Gold

Gold is one of the two basic currencies of the game, the other one being diamonds.

You need gold to improve, upgrade or acquire almost any feature of the game.

Getting Gold In-Game

You can get gold through many game activities. At first you'll have limited means, but as your Kingdom Level increases you'll have more and better options. The maximum daily yield can be increased until ~40.000 gold/day for free users, with substantially more available for diamonds.

In rough order of yield:

Level Taxes: Your main source of income (after Kingdom Level 15 or so). Depends on your progress through the main Tower defense storyline, how do you use your energy, and specially the resources you intend to get. Expect between 5000-20000 gold/day. See below for details.
Manor (Money Tree): At low Kingdom Levels (from KL 6 to ~15), this may be your main source of income. 800 gold per patch per seeding (8h CD), thus 3200-4800 gold/day for 2 patches, 4800-7200 gold/day for 3 patches, etc (this assumes 2-3 seedings/day, this can be improved with VIP level 1 and 7).
Prison (after KL 35)
- Battles (8x ~300-400 gold = ~2400-3200 gold, plus random plundering)
- Ransoms (8x 50 gold = 400 gold/day, assuming you never ransom a prisoner more than once)
Fighting Pits
- Battles: 1000-3000 gold/day (100 gold per failure, 300 gold per victory)
- Reward every 3 days:
  - Guaranteed 1750 gold (583 gold/day)
  - Depending on rank: 1000 gold/day requires rank 500, 1500 gold/day requires rank 70, 2000 gold/day requires rank 20, and below this, while it certainly soars to the 6666 gold/day for Rank 1, it's not practical for most.
Kingdom Tax: Depends on your Kingdom Honor and Level Stars. ~1000-1500 gold/day, unless you pay Diamonds or are of VIP level 2+. Through heavy diamond spending and some luck, this can rise to be your main source of income, see VIP for details.
Poseidon Boss (once a day) (1000-1300 gold, as it's likely you won't rank)
Blitz Acres (From 75 gold/day if you can beat Wave 5, up to 2150 gold/day if you beat it in full)
Online Reward (1000 gold/day)
Reward Center
- Daily Spin: Up to 600 gold/day, very probably less. It's replaced by Daily Login Reward, with no gold.
- Daily Check: 2000 on day 22 and 5000 on day 28, averaging 250 gold/day
Daily Tasks (300 gold)
Tavern (variable, the almost guaranteed amount is 300 gold)
Day 2 and Day 6 of Daily Grind
Crown Port (Ports)

Level Tax Yield

The yield for level taxes has been calculated this way:

Average yield for Hard difficulties for each zone: Grasslands 70 gold, The Tundra 120 gold, Lava Plains 170 gold, The Desert 220 gold, Marshlands 270 gold, Oblivion's Cave 320 gold, Deep Sea 370 gold and Mech Warship 420 gold. Heroic levels yield, on average, 30 gold less each.
Assuming (for a conservative estimate)
1. You farm levels for resources first and gold later
2. You spread your taxes claims half on Heroic and half on Hard levels (both giving the same resources)
3. You do not prioritize the best available levels (possibly because you still haven't unlocked them)
4. You use all of your free daily energy (80 level taxes each day)
5. You need to spread your level taxes among the three zones that yield the resources needed to upgrade your towers and magic skills (e.g. Grasslands, Tundra and Lava Plains for towers level 20-30), let's say a third on each zone, which means 26/day on each zone, plus 2 on the richest zone).

Thus, the daily average yield you may expect will be:

5000 gold when you only have Grasslands
7500 gold when exploiting Grasslands and The Tundra
8500 gold when you are exploiting Grasslands, The Tundra and Lava Plains
12500 gold when you outgrow the Grasslands (towers and magic skills over level 31)
16500 gold when you outgrow the Tundra (towers/magic all over 41)
20500 gold when all magic skills and towers are over level 51.
24500 gold when you outgrow the Desert (magic skills and towers are over level 70)

The previous numbers are conservative estimates. If you prioritize the best levels for each zone, and Hard difficulties over Heroic ones, you are likely to get much more. This difference is ~1000 extra gold/day using only Hard difficulties, and another ~1000 extra gold/day if you limit yourself to the best 3 scenarios of each zone instead of all of them. This increase is the same for all zones and combinations, owing to the linear nature of rewards increase (5 more gold /level, on all difficulties)

Level Tax yield, more gold

Another way to optimize the yield of your level tax is to limit yourself to the best possible scenario you can reap taxes from (to get gold) and leave all resource harvesting to the Supply Station. This works best once you've unlocked the second (and third) slots and have progressed beyond Lava Plains on the main storyline.

For basic resources (from Grasslands, The Tundra and Lava Plains) you are limited to a best option of 46% of tower resource, 29% of magic resource (thus 0,75 resources, on average, from the best level tax)
For Quality resources (from The Desert, Marshlands and Oblivion's Cave) you are limited to a best option of 19% of tower resource and 13% of magic resource (thus 0,32 resources, on average, from the best level tax).
For Pure resources (from Deep Sea) you are limited to a best option of 21% of tower resource and 13% of magic resource
Basic resources are worth 100 gold each on Supply runs (assuming you have enough gold and are upgrading towers and magic skills at the appropriate rate so no resource is left unused). Quality resources are worth 700 gold each (5x basic resources plus a forge)
Consequently, if you can reap taxes from a scenario that offers more than 75 extra gold against the best source of a basic resource, you're better off claiming taxes from there and obtaining resources from Supply Runs. The same for Quality resources, but a difference of 224 gold is needed, so it's harder. Anyway you'll probably have too many resources by the time you reach this point (for the 9x daily Supply Runs you'll need to send to optimize Soul Shards for rank).

Example: I need Fairy Wood to improve my towers. The best source of the resource is Furnace Valley (Hard) on The Tundra, at 135 gold for a 46%. But I have already beaten Mirage Sands (Hard) on The Desert, which gives 215 gold. I will be better off claiming the Desert tax. I'll have more gold (and I can also spend it on other places, if I find I need more gold), plus I'll be slowly gaining a Quality resource that is much harder to substitute (and since the Quality resources cannot be stolen through the Prison, I can stockpile it safely)

This method is both useful and simpler than previous one (if the highest I can reach is Mirage Sands (hard), my best daily Level Tax results will be 215*80 = 17200 gold). The obvious disadvantage is that it requires more visits to the world to spend your energy, little by little.

Buying gold

As with diamonds, you can also purchase gold with real world money. The basic deal is 1000 gold for $2, or 500 gold/$, and the best deal for bulk buying is 130000 gold for $200, or 650 gold/$. Though it seems like much, the price tags on game upgrades are also high. So the price of gold is really high. Moreover, the game allows you to get enough gold through game mechanics to fulfill your needs (though it may limit the speed of your upgrades), so there is no real requirement to buy gold, ever.

Keep in mind that a healthy starting Kingdom may need as much as 5000-10000 gold each day (depending on planned upgrades) before running into other constraints such as resources. Once you start planning to upgrade rank, and thus start launching 9 Supply Station runs each day (800*9 = 7200 gold), plus tower upgrades on level 40+ costing 3000+ gold each, plus almost-required Forge use for higher-level resources, plus... let's just say that you may find you need each coin from the ~20000/day you may earn for free. And you can be there on a couple of months of gameplay.

You can also buy gold as part of other paid packages. We will not consider them here as their availability is not constant nor guaranteed.

Using Gold

You need gold to (in roughly descending order of importance)

Seeding at the Manor
Upgrade, research, talent towers on the Engineering Quarter
Upgrade, research magic skills and harvesting essences on the Magic Ward
Train your heroes on the Training Grounds

Upgrade heroes' rank: 25000 (C+), 37500 (B), 50000 (B+), 100000 (A)

Get supplies through the Supply Station
Recruit soldiers on the Barracks
Upgrade the Barracks building and Rural Camp itself.
Get jewels and Tear of Shiva through the Jewel Crafter
Enhance equipment, researching quality and increasing tiers
Augment heroes in the Hall of Heroes
Synthesize at the Forge
Research Technology at the Castle

For some of these activities, you can substitute with diamonds. Also, many of them require some items, usually resources, that you must gain from other game activities (some of them require expending gold), or buy from the marketplace (with diamonds, except during the Black Market (now Mysterious Store) regular event).

If you find you need to burn excess gold (specially after you unlock the Prison at Kingdom Level 35), and you cannot spend it on the preferred big upgrades, you can burn it on

Castle technologies (1.000 gold increments) (see below for the Gold to Battle Power ratio discussion).
Jewel Crafter (200 or 400 gold increments) (see Jewel Crafter for optimization and strategy)
Augment (100 gold increments) (the most versatile option, at least while the chance is over 40%, see below for the Gold to Battle Power ratio discussion).

Gold to Battle Power Ratio

This is a discussion on efficiency/optimization of resources. You do not need to know anything on this section to play.

Basics

Most of the upgrades of the game, if not all, will increase your Battle Power. Since it's (almost) impossible to max out everything, you must spend your resources (both items and gold) wisely, trying to make the most of every one. But not all uses of gold are the same, thus it is needed a way to measure the relative efficiency of each use. We've used the Battle Power (Gold/BP) measure as a proxy for this because it's easy to measure and compare across all features of the game.

Exclusions

Recruiting soldiers at the barracks is not measured because it doesn't have a direct effect.
The Manor would be excluded except for the Royal Wheat. See Hero Experience for a discussion of this topic.

Extreme Upgrades

These are the upgrades that offer extremely low rates, lower than 10 gold/BP, and they all require Dark Soul Crystals. They are:

Heroes Skill and Talents (out of scale because they don't cost gold)
Research Tier 4 skills (except Facebook): Costing 5 DSC each, they offer ~4 Gold/BP
Research Tier 3 Towers: Costing 2 DSC each, they offer ~6-8 Gold/BP
Rank: ~6 gold/BP

Major Upgrades

Ratios from 10 to 20. They are limited by resources.

Magic

Magic skills upgrades offer ~10 gold/BP. The bad news is they only affect tower defense on full levels (every 4 upgrades)

Towers

Towers upgrades offer ~20 gold/BP, and they offer a measurable, instantaneous upgrade.

Augment

Augment has rates comparable to towers until the 75% success rate.

Medium Upgrades

Ratios from 20 to 50

Augment

Augmenting offers a good rate until the 50% success rate.

Enhance: Main article: Enhance

Enhancing an equipment piece only increases your Battle Power while heroes are actually wearing the enhanced piece. Because of this, it is a suboptimal choice in most situations except for the pieces that require Grindstones. After you need Grindstones (for Specials, level 16+, for Elites, level 11+ , for Wicked, level 6+) the Gold/BP drops to an excellent ~20. This is comparable with Tower Upgrades, but...

First you have to dump a bit of gold at the base 50 ratio,
Sooner or later your heroes will outgrow the equipment

Tears+Jewels

Until you complete all your 16x Lv.4 Jewels (and this may take a while), the combined BP increase of Tear of Shiva and Jewelry will be able to compete with the Tower upgrades. Although the randomness makes this effective only in the long run, with many temporary inefficiencies (while you accumulate more jewels from one color than other, for example, due to random fluctuations).

Heroes

Leveling up Heroes depends a lot on upgraded Training Grounds slots. At the basic level, their ratio becomes 40 at level 10. This threshold is at level 20 with a 20% upgraded slot, level 26 with a 40% one, and level 41 with a 60% one.

It's very important to upgrade them in order to increase more BP.

Minor Upgrades

Ratios 50 and up. These should be done to burn excess gold, but don't do these while you can do any of the above.

Augment

Augment is competitive with the Tear of Shiva until the 40% success rate.

Enhance

Enhancing pieces which don't require Grindstones has a ratio of 40-50 gold/BP. As said before, you should only spend gold in the pieces you intent to grow until their Grindstone level. To spend gold at ratio 50, you'll be better off buying Tear of Shiva.

Tears of Shiva: Main article: Tear of Shiva

Tears of Shiva, as seen above, can be converted to BP. The ratio is always 2BP/tear (50gold/BP) except, as seen before, with the level 2 soldiers upgrade (which can absorb a non-trivial-at-all amount of up to 3.840 Tears). But with the exception of this, using the Tears to learn Tower Upgrades at the Engineering Quarter is most useful, because not only do you get 50 Gold/BP, but you also improve the tower defense part of the game. This is only from Kingdom Level 30 onwards, mind.

Jewels: Main article: Jewelry

Even through the best use of Jewel Crafter strategy, each Lv.4 jewel has a ratio of 60, at a minimum (10k gold/162 BP), and can be as low as 100 (16k gold/162BP). Lower level jewels may (by luck) be better (and best of all if won for free on the Tavern), but on general terms, the best ratio is on Lv.3 or Lv.4 jewels. Above this, BP increases by 3 on each step, but price increases by 4 (the stones needed to forge a higher one), thus the ratio is ever-worsening.

However, since with those 10.000 gold you also get 100 Tears (thus 200 BP) in addition to the 162 BP of the jewels, the effect of those two being linked is far from negligible. 10.000 / 362 = 27.62, which is a rate comparable with Towers.

For higher level jewels, we can see the combined cost approaches asymptotically to the ratio of the Tears alone, 50 gold/BP Level 5 jewels: 40k gold, 80 gold/BP. +400 Tears, 40k/880 = 45 gold/BP Level 6 jewels: 160k gold, 105.66 gold/BP. +1600 Tears, 160k/3305.66 = 48 gold/BP Level 7 jewels: 640k gold, 142.22 gold/BP. +6400 Tears, 640k/12942.22 = 49.45 gold/BP Level 8 jewels: 2560k gold, 189.63gold/BP. +25.6k Tears, 2560k/51389.63 = 49.86 gold/BP

Castle Technology

A quick word: don't (or do very little). Basic castle technology offers 50 Gold/BP, while Moderate and High offer 45gold/BP. See on Castle page and Honor Stars strategy section why you shouldn't use Moderate and High upgrades until you unlock all the heroes, so you're stuck with 50 gold/BP rate.

As for the ratio, not only do almost all of the Engineering Quarter and Magic Ward upgrades offer better rates: your gold will be best used in the Training Grounds, Manor, Barracks and almost anywhere else.

Castle vs Tears

To burn excess gold you can use Castle technology, or convert it into Tear of Shiva (see that page), and train Soldiers at the Hall of Heroes. At the first level, you will get 10BP for 5 tears. That is 500 gold/10BP, the same rate as the Castle. On the second one, it will be 16 Tears for 35BP, 1600/35=45 BP/gold. The next rates go back to 50gold/BP. So the most efficient way to increase your BP with your excess gold would be to increase all Soldiers to the second level, and then keep increasing them until the next level. After this, Castle Technology may be preferable because it only requires gold, and not increasingly rare resources.