BEST! Spike Trap Guide Ever Exist, (PS: noob friendly with popping pictures & coloured text)

To kill archers/builders: 9 tiles deep with spikes at the bottom
To kill everyone: 15 tiles deep with spikes at the bottom

 

Yes that's right, weeksy. Tested it by my own too

 

Does putting spikes on the walls actually prevent enemies from sliding against them?

 

If the enemy pushes into them in an attempt to wallslide and prevent the fall damage, then they take spike damage instead. It's important to have spikes all the way down the sides, to prevent the enemy from any tricky wallslides that wind up with a live digger at the bottom of the pit.