There are three types of terrain in the game: Plains, Swamps and Walls. The terrain of a room affects everything in it, from pathing, to where you place your structures, to where you set up defenses.
Plains are the standard walkable tile type in the game. Plains tiles cause 2 fatigue per bodypart
Swamps are a walkable terrain type. Swamps cause 10 fatigue per body part, five times more than plains. This causes moving through swamp tiles to be much slower than through Plains tiles.
Swamps pose interesting tactical and strategic challenges for players. For a creep to be able to move once per tick on a Swamp tile, it will need 5 MOVE parts for every 1 of another type.
Militarily, swamps provide players with a dilemma- creeps are unable to maneuver properly when fighting in swamplands. This can provide some players with strong defensive positions, for example when attacking creeps must path around or through swamps, breaking up their formation or exposing them to enemy attack for a longer period of time.
Natural Walls are a non-passable terrain type. They cannot be harvested or destroyed. With the exception of exit tiles, rooms are bordered by walls.
Walls can cause a room to be nearly unusable due to choke points, provide a natural defense or even take up 90% of a room. Wall placement will have a major effect on room setup, or whether a room is even usable at all.