This killbox design is deadly to anything organic. Even fire-resistant imps can't withstand the 1000C+ temps inside.
This "firebox" is so effective because
1. The turret is irresistable
2. The hay floors are incredibly flammable
3. Open doors still trap heat
By combining these three things together, we force RimWorld's Standard Raider AI to walk into a self-igniting oven, cooking themselves to perfection until they break and flee. It's really quite awful and might give you nightmares. We'll see.