So i was building a platform-like thing on the side of a cliff, but I discovered at night that mobs could easily just walk off the cliff and fall on to enact their sadistic revenge for their fallen brothers upon me. So i walled up the top of the cliff with cacti, which stopped everything but spiders. They hurt themselve in the process, but still got over. Now I really don't just want to build a wall with a lip because that looks pretty ugly.
So what do you suggest for preventing spiders from crossing?