Constructing, as well as demolishing is important (well, not necessarily the latter). Buildings have different costs and building times (measured in ticks), for example; the fastest, a silo, needs only 2 ticks while one of the slowest, a University, takes 10 ticks. A different amount of gold, iron and wood are needed for different buildings. More buildings are available to build as your empire advances. Bruce "gets it done" Hanes can also demolish buildings for more land at a small cost.
(Introduction by beeasyonme - necron lord)