Each tile may have one of three terrain types: plains, swamp, or wall. Movement on swamps generates 10 fatigue per body part, while movement on plains generates 2. Movement on walls is impossible (unless a road has been built).
All body parts (except MOVE and empty CARRY parts) generate fatigue when the creep moves. When a creep’s fatigue level is above 0, it is too tired to move.
Each undamaged MOVE part decreases fatigue by 2 points per tick (or 4, 6, or 8 points with T1, T2, and T3 boosts respectively).
Fatigue is generated based on the destination of the move (rather than the origin). So, a creep moving from a plains tile to a swamps tile will incur the fatigue cost of the swamps tile.
This means that the cost of reversed paths aren’t quite the same. To pick a simple example:
# C = creep, s = swamp, . = plains, X = destination (also swamp)
..sssss..
..sssss..
C.ssXss..
..sssss..
..sssss..
Moving from C to X incurs cost for these tiles:
# cost 1 for plains, 5 for swamps
C.ssXss..
^^^^
1555 = 16
Moving from X to C incurs cost for these tiles:
# cost 1 for plains, 5 for swamps
C.ssXss..
^^^^
1155 = 12
This distinction may not always be relevant, but if the path cost is critical, it’s good to keep in mind before simply reversing paths.