Garrison
Garrison is an adventure map object, that can withhold creature troops. An enemy hero can not pass the garrison without fighting the armies inside. Garrisons can be aligned both horizontally and vertically, and are typically used in narrow passes to create obstacle, that can not easily be passed.
It should be noted, that mapmaker can set garrison's troops to be impossible to remove. This means, that if a hero transfers creatures to garrison, they will stay there permanently.