Every acre produces 10 gold/hr regardless of what is built on it.
Each structure costs 500 gold, before hero type effects are factored in. These are the building types available to you:

Mines

Mines are the driving force behind your economy. Each mine provides 30 additional gold/hr (40 total) for your kingdom. Add some trade agreements, and watch your economy flourish.

Barracks

Barracks help to reduce your training costs. The higher proportion of your land that is covered by these, the lower your recruiting and upgrading costs.
% reduction in recruit/upgrade costs = 2 times the % of land covered by barracks
maximum cost reduction = 50%

Forts

Forts augment the defensive capabilities of your standing army. Cover more of your lands with forts, and your defense should improve.
% increase in defense = 1.5 times the % of land covered by forts
maximum increase in defense = 45%

Training Grounds

Well trained troops perform better on the battlefield. Training grounds act as the opposite of forts, providing your troops with an offensive bonus.
%increase in attack = 1.5 times the % of land covered by training grounds
maximum increase in attack = 45%

Amplifiers

Even the best magicians need some help once in a while. Amplifiers increase the natural strength of certain targetted spells.
% increase in spell damage = 2 times the % of land covered by amplifiers
maximum increase in spell damage = 50%

Barriers

Tired of getting blasted by spells? Constructing magical barriers can effectively dampen your lands with a defensive matrix, weakening offensive spells targeting your kingdom.
% increase in magic defense = 3 times the % of land covered by barriers
maximum increase in magic defense = 75%