Ranger "Refugees" - [5th campaign, 1st mission]

I can't figure out how to withdraw troops from the garrison. You can remove them from the hero, but it's impossible to do so from the garrison. Necropolis heroes start attacking from day 3 (I manage to repel 1-2 attacks). If you place a hero in the fortress, only he and his army fight, i.e., it's unclear where the garrison troops are. I've been struggling with this mission for about 4 days.

How do I withdraw troops from the garrison (please explain in detail)?

P.S. Game version 1.2