Bank (Building)

For the Wraith skill "Bank", see Bank (Skill).

The Bank is a building available through the Forge, which serves only to generate gold, generating an amount of gold at the beginning of each day, scaling with it's level as well as multipliers from skills and research.

Skills
Bank (Skill): The Wraith skill increases bank income by up to 175% at level 85.

Research
Guild Bank: This research option gives a 1% income bonus to the bank for each statue level.

Stock Market: Bank income increases by 2% for each achievement earned.

Strategy
The bank is the principal means of generating gold in Dungeon Defense, in higher difficulties and later levels, it's gold generation far outstrips that of the statue as well as combat gold. Managing and maximizing the value of your gold for the purposes of upgrading dungeon buildings as quickly and to as high a level as possible is core to surviving the largest number of days possible.

The cost of each bank upgrade increases exponentially, but the income from upgrading the bank increases linearly. This means that, as the difference in cost increases with levels, the benefit you gain in the form of increased income will take a larger number of days to repay the cost of the upgrade and start actually paying you back. For this reason, the skills and research which increase bank income or especially reduction in building costs are vital to getting the most out of the bank.

With the goal of reaching the highest number of days survived as possible, upgrading the bank must be balanced against upgrading the dungeon walls max HP and regeneration, as these are what will ultimately determine how far you can progress. It is generally advisable to hold off on upgrading the dungeon wall for as long as possible to start, and upgrade the bank/statue instead. Upgrading the wall early provides no benefit if the enemies never reach it in the first place, so spending gold on it essentially does nothing for a large portion of the game, whereas the bank will be given the time to pay itself off. On the other hand, if you spend all your time upgrading your income, then come to a point where you cannot progress easily before getting overrun, what potentially massive income you may have from the bank cannot be put to good use, and all the gold you've sunk into upgrading it isn't being useful, meaning you'd sunken money into an investment that doesn't have enough time to provide returns. There is a point where, forecasting far ahead, will leave you with an optimal time to stop upgrading income and start upgrading exclusively the wall HP and regen.

Leveling the statue increases bank income with the Guild Bank upgrade, though which upgrade provides the greatest gold-to-income efficiency will depend on the relative levels of each. With a much higher bank level, the 1% increase from the statue may increase your income more than another bank upgrade.

Note that accessories which give "+% Gold Earned" do not affect the bank or statue, and only apply to gold earned from combat, similarly to the Skeleton and Living Sword skill Bounty Hunter.

It is currently not known (or at least nobody has bothered to research it) whether bonuses from research upgrades and skills etc. stack additively or multiplicatively.

Builder Skill
The Ogre and Orc skill Builder is especially helpful here. At maximum level, Builder reduces the building costs of all forge buildings by 75%. Buildings costing 25% of their original value means that you effectively quadruple the efficiency of gold spent on upgrades. Additionally, possessing the Necklace MK-14 which gives levels to all skills will increase the builder skill, with each level giving an additional 1% to price reduction. At level 15, the amulet will give +3 levels, resulting in -78% price reduction. All that upgrading on the amulet may seem costly for the last 3% reduction it gives, but buildings costing 22% of their original value gives your gold 4.55x value when upgrading, compared to 4x at 75%.