Terrain
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
Plains are the standard walkable tile type in the game. To walk on plains tile once per tick, a creep needs to have half the size of it's body be MOVE parts.
Swamps
Swamp is a terrain type that is walkable by creeps. Though it is walkable terrain, it has an increased movement cost compared to plains. This causes pathing through swamps more difficult than Plains.
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.
Walls
Wall is a regular non-passable terrain type which makes boundaries between different players. It 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 how you set up your room, or even if a room is usable at all.
Controllers, Energy Sources, Power Banks, and several other objects in the world are embedded in a wall tile.