The "hole" in the wall is covered up and there is no real way to sneak into it the way you're supposed to - only way in is through the front gate

If anyone has the same problem:
the AI doesn't seem to be able to move inside the castle; so you actually cannot really escape with the prisoner.

But there is a way: tell the prisoner to follow you (he'll stay right where he is), drive the APC in reverse directly to his prison wall at the backside of the castle and tell him to get into the vehicle

-> the prisoner will then move through the wall into the APC (or at least outside the dungeon, as he once was near the castle, where I then could pick him up).

Миссия просто бомба! это лучшая миссия из всех миссии, которые я играл, всё работает отлично, пленник послушный, с модами на улучшенный ИИ вообще хардкор, раз 15 умирал))

пленником управлять нельзя, в бтр он не садится