I am worthless beyond hope.
Join Date: May 2004
Location: Inner Swabia. If you have to ask twice, don't.
I modified the wall towards the surface entrance to the passive trap after a creeper blew up part of it, and I was attacked by a spider. I actually don't think that a wall that isn't spider proofed on the town's side plus a trench does anything to prevent them from jumping across the chasm. In fact, I am absolutely sure it doesn't since spiders can grab walls while falling without taking injury. So please adjust your designs accordingly -- adding a lip onto the wall is sufficient. Also, please light up the top of the wall, as mobs can spawn there.
Note that your access to the surface entrance has been blocked. Feel free to add a door but do not change the 'normal' state of it being closed off since mobs were gathering at the entrance and beyond while I was working on 'my' side of the fence. In addition, I will be adding spider proofing to the edges of the trap on that side since there is absolutely nothing to prevent spiders finding their way into the trap and causing problems for archers working in the middle.
All of this said, it would be less work to just light up your town so hostile mobs don't spawn, but while it's your choice to put your structures and avatar's health at risk, just be sure no one else's are at risk by your interesting design approaches.
To the general audience: Should our standard be that new construction is lit up or designed to prevent mob spill over to existing structures/safe areas? I would have thought this was obvious, but who knows?