Gold

From The Five Pillars

Gold – the yellow shinies that makes the world go round!

Gold is generated by population. The more houses you have, the more the population can grow, and the more income you will receive. Gold is required for the upkeep of most buildings, heroes and many units.
Several items can generate more money: Tax Bill, Chest of Jewels, Manual to Usury, Fool's Stone, Safe and maybe some I forgot.
Several spells can also generate money: Alchemy, Return of the Magpies, Elemental Conversion.

Besides upkeeping buildings and units, gold can be used on the Market to buy items, spells, Heros, and units. It can also be used for donations to the Oracle. The Oracle is an unpredictable old woman that spits a lot. It can howerer reward you with troops (Changelings), extra Spellevel, Items, Heroes, an enchantment or even gold. Sometimes she just steals more gold.

If you run negative gold upkeeps and reach 0 gold in your treasury, units, heros, and even buildings will be sold or released from service (most expensive first), to try to restore a proper balance to the treasury.
WARNING: This includes your forts! If you loose your last fort your mage will dissolve in the void and all you can do is reincarnate.

You must keep your treasury well guarded with many troops, otherwise your gold and items may be looted by your enemy! (Several thousands of troops are needed to notice a sneaky army sent out to loot.)