Land is the most important type of terrain in Rise of Nations. Firstly, because Cities, which are the backbone of a nation's production and economy, along with most buildings that come thereafter, require open, unobstructed land to be built upon. Secondly, because every other terrain feature (except for Water of course) where the vast majority of Basic Resources are gathered, such as Mountains or Forests, can only be found on land, as well as all but two Rare Resources.
Land comes in all sorts of varieties, from tundra to desert, depending on the Terrain Set. This variation however is purely cosmetic by nature and makes no difference in resource yields or any gameplay factor.
Open land may also present irregular height throughout. Land open but unleveled is not an obstacle for constructing buildings, not even in the most steep slopes; a chosen building site with irregular height will automatically level the irregularities on the terrain, and even if the building is cancelled or destroyed, the change in the land level will be permanent.