Tactics for the Swamp (Fortress).

You won't be able to pull off that trick. Figure out why.

Well, it's clear that it won't always work. Some enemies can't be stalled, some will immediately attack with high speed, and there can be various other situations, such as ranged units, and so on. The main idea is usually to meet the enemy on your side of the field.
I had a different problem when playing with a friend on an L-shaped map without an underground. He took a Castle, and I took a Fortress. He started with an Ogre, and since he later used that tactic, I placed one stack of archers in the upper corner before the battle, protecting them with other units, and did the same in the lower corner, so it didn't make sense for him to advance; he just kept shooting. With this setup, everything became more complicated.