Stacked units should fight one at a time, so Thermopylae can still happen. It adds a lot of depth to the game--particularly at higher levels--when a single unit

Stacked units should fight one at a time, so Thermopylae can still happen. It adds a lot of depth to the game--particularly at higher levels--when a single unit can can drastically slow down a large group of units, just by staying on defensible terrain in an isthmus.