With the lack of traps, here’s my suggestion. Pitfall. Pitfall can do either of 2 things:
You can make it so that it only goes away once it’s filled up with units, and increase the depth with each level.
Make it so that it fills in over time, and increase the amount of time it takes with each level.
Would be a pain in the rump for high morale units like Ogre, Arblaster, etc., who use a lot of morale but aren’t very big.