So I kited a Giant to a watchtower and I stood inside and cast spells on him, he just waited by the entrance and let me burn him because he is too large to enter.

However I noticed that a mudcrab followed us, and after the Giant died, it was the mudcrab's turn. The little bug did NOT enter. He just sat outside and waited to die. He is small enough to enter, so what is the reason mudcrabs ( and probably wolves as well) can't get inside?

Is the game so poorly coded that only humanoids can enter the towers?

Creature path finding AI? Not a real problem really. I have more problem with a giant just standing and being killed because he's too dumb to realize that he can't get to the attacker and needs to run away.

Crustacean. AI pathfinding scripting fail is all.

Whenever I try to trap a NPC or mob against terrain it flees behind cover or circles around back

The big Mudcrabs are very big. Bigger than you. Check it out in 3rd person. The small tiny ones should be able to get through the door, though.

It's because they are solar powered.

They will use another way if one exist, if not like the giant and the watchtower he could not enter they don't move away.
Ranged npc will use cover.

I've tried that against Giants they run away as soon as they realize they cant get to me.