In Rise of Nations, Terrain refers to the different physical features of the Game Map. Terrain is not only of great strategic value, it's also fundamental in a nation's economy and its possibilities of victory. The frequency, or althogether presence of land, water, or terrain features in any given map depends on the type of map or Scenario being played. The variations in terrain which can be found in the game are the following:
Land is the most important type of terrain, being necessary for Cities, most buildings, and the efficient gathering of many basic and rare resources. Many varieties of land exist in the game, but the difference is purely a cosmetic one, having no further impact.
Besides land, maps can also feature one or several bodies of water, regarded in the terrain editor as Ocean. A great source of Oil, and the only source of Fish and Whales, any map with a body of water provides an entire addition of strategical possibilities and priorities in the form of Naval Units.
Mountains are the primary source of Metal, a fundamental Basic Resource, which can only be obtained from Mines, which in turn can only be built on Mountains or Cliffs. Mountains also make impassable barriers for land units; as such they can create choke points which can provide sound strategical opportunities.
Cliffs are, much like mountains, an impassable barrier for land units, although unlike Mountains, enemy ranged units can shoot to objects in the other side. Their impassable nature however means that they can have great defensive value, especially from the side of the Cliff with higher ground, which gives a bonus to damage. Mines can also be built on Cliffs, and only one Mine per nation can be built on a Cliff, as is the case with Mountains.
Forest is the terrain where Timber, one of the most essential resources, can be gathered. Like Mountains and Cliffs, Forests provide physical barriers impassable by land units, Iroquois Scouts being the only exception. This means that Forests also work as choke points, much like Mountains and Cliffs, and even more so if accompanied by them.
Rivers are a strategic terrain feature found on some maps. They cannot be built upon and units crossing a River move at half speed, and take twice as much damage, making Rivers both extremely useful defensive barrier and a double-edged sword. Unlike Ocean, no Naval Units can navigate through a River, nor can Docks be built on a riverbank.
Rocks are found in open land, and nothing can be built on them in the early game, making them useless at first. From the Industrial Age onwards however, some resources become visible on the map, and most Rocks may end up containing a resource, most importantly Oil. Rocks don't guarantee the presence of Oil, but Oil patches on land can only appear in Rocks, the only other source being Industrial age Rare Resources, which also tend to sit upon Rocks, and Oil deposits in the Ocean.