Thank you for the feedback, and glad you like the art! Enemies will only attack walls once they've been given an attack order, which currently happens either when they're damaged by a knight or when the attack timer at the top of the screen reaches 0. So even if a wall is built right next to them, they don't consider it an attack target if they're not in attack mode. Adjusting this is definitely on my to-do list for version 1.0.