The Forest by Endnight has some really cool traps. “Happy Birthday” trap is basically a spring loaded spiked wall, and the log fall traps are as simple as it gets. Not sure how they’d be incorporated into the game, but it would be great.
Not a defense, per say, but more offensive: Battering rams. Lower bomb damage so that they’re only ideal for things like vaults and boxes, and use battering rams for base raiding. The defensive play comes in with knowing where your choke points are going to be so you can defend your gate from rams.
Another defensive idea: Machicolations, arrow slits, and murder holes that archer thralls can actually use (Basically code them with 1-sided projectile collision)