So.........

[jimkehn]

Why didn't these guys retreat back into the fort?????

Loemender Map.

[llamabeast]

You can't retreat into a fort.

[jimkehn]

Oh. I always thought if you came out of a fort to break the siege and were defeated, you would retreat back into the fort. Guess I was wrong.

[vfb]

Any remaining forces in the castle lock the gates, raise the drawbridge, lower the portcullis, and throw the keys down the well. Or the bottomless pit, if they've installed one. Cowards!

You can retreat into friendly neighboring provinces only. Which doesn't work if the only neighboring friendly province is invaded the same turn by hordes of gibbering mouthers like I see happened in your game.

[llamabeast]

Yeah, it's so you can't invade from a neighbouring province with a big army and set everyone on retreat, so the whole army ends up inside the fort.

[jimkehn]

Yeah...I just read it in the manual. Terrible, horrible loss. I didn't mind losing the guys in the fort so much as the guys coming from outside the province. Had those R'Lyeh fools not taken my only existing adjacent province, then at least all those commanders coulda survived to fight another day, to say nothing of the line troops.