Garrisons represent castles and military bases. And it's weird that when a battle starts, the invader can freely deploy it's units in the defender fortifications without combat. It gets even more strange, if Garrisons are clustered (to make a big Castle that can cover multiple units), but when the battle starts, part of this big fortification is attackers deployment, so the fortifications don't really work, especially since the attacker has the first turn.
The only cool exclusion could be stealth armies, that could infiltrate the fortification due to their skills.