In one of my missions I saw something strange: Chryssalids were unable to enter a small truck's trunk with just one panel broken. They even didn't attack a person standing right in front of this panel. (My only survivor, a heavily wounded sniper, was able to kill 6 Chryssalids.)
Why couldn't they attack me here? Are there any other limitations?
I also think Sectopods can't use stairs/drainpipes, and I've never seen a Berserker use one either.
EDIT: I was wrong, Sectopods can climb ladders (or jump?). Otherwise I have no idea how next screenshot can happen as at previous turn he was downstairs.
Best Answer
The reason they were unable to do anything is because the enemy needs to be on an adjacent square on the same plane as the target to use a melee attack. By standing in front of the only broken panel, you were blocking them from entering the same plane as you. In the same vein, you can prevent aliens from leaving a room by creating a meat wall of your operatives on all of its exits - if the doors are closed, they will be unable to attack (having no LoS) or open the doors, since they can't route to a point past the door.