I am near certain that it is impossible for the skeletons to spawn in an area you can't reach. If I remember correctly, there is a hole in a wall or gate that is easy to overlook and not notice that you can walk through where it will give you access. One trick to find it is if you have any fighters with you, you can give them the "Charge" command (key "C") and they will run directly to it and you can follow them. You can also send me the save file to chezjfrey@hotmail.com and I will find that hole that I mentioned; I'm pretty sure if I am placed in that dungeon I will eventually remember how to find that spot.