If a unit is at least partially within marshland, it suffers a -1 penalty to its Movement value. Marshland is dangerous terrain for all units, other than those with the Skirmish special rule. Cavalry, monstrous cavalry, chariots and shrines that enter marshland fail their Dangerous Terrain tests on a 1 or 2, rather than a 1.
Below is a selection of different marshlands that you can choose to include in your games: