A unit stack limit might be easy, or hard, or in the worst case scenario it would appear easy but then cause all sorts of nasty little problems. It would be easy to block movement onto tiles with too many units on them, like I already did for the trespassing restriction. Though I'm concerned the AI wouldn't be able to handle that. I can already anticipate one issue, because it's bitten me before, the movement of AI escorting units and their escortees. If you limit tiles to three units, and then have a tile with two units that a spearman+settler pair is trying to move onto, that could create a problem since the move would be allowed at first but become disallowed half way through, after the escorting spearman moves.