Scripts

It won't crash. At most, it might lag during map loading.

Sometimes, it's easier to use obvious workarounds than to overcomplicate things. To solve this problem without regions and triggers (and generally, without any settings for a garrison, even without assigning it a name), you'll need a brief introduction to OOP, a discussion of several new, complex functions, and an understanding of tables in Lua.

Jewilly, if you're interested in an elegant solution, I'll just say what it's based on:
Using the IsTilePassable function, you can check the location of blocked cells relative to the center of the object (for a garrison, one is above, one is below). After identifying the garrison's orientation, the hero is simply moved to the other side using SetObjectPosition with teleportation type 0 (without animation) if the hero's owner is not a banned player (all of this is done through a touch trigger).

And for aesthetics, you can create a "Garrison" class with methods like BanPlayer, UnbanPlayer, and other similar things if you want to add any effects, additional messages, and so on.