What can I do to secure my towns/castles/hovels from climbing spiders?

This answer has gone through many, many edits, so I decided to overhaul it. (Minecraft Forums user Thescorpi0on did a number of tests on this.)

Things that will stop spiders:

• A lip extending off of your wall
• Ladders on the top of your wall

Things that won't stop spiders:

• Torches
• Signs
• Cacti (it will injure them but they can climb up them)
• Glass
• Sandstone
• Ladders going all the way up your wall (they will start climbing at the bottom all the way up)

In addition to the above methods, I've found that a cactus wall around my tower ends up murdering all the spiders. They climb to the top of the cacti and sit there. They can't get to the tower wall because of the necessary one space gap involved with a cactus wall and proceed to die to the cacti spikes.

My house is literally made 100% of water. Try climbing that you damn spiders!

Use my patented Spider Dissuasion Device.

Ready to dissuade:

Dissuading:

250% more amusing than lipped walls alone.

(May not be more amusing. May cause damage to livestock, buildings, operatives, sticks, handbags, weddings, and Kuala Lumpur.)

New update - Spiders can now climb anything but cactus, the lip trick still works though.