Mobs can climb ladders, but they're not smart enough to do so if you're not above the ladder, and at the proper angle for them to catch the ladder while walking towards you.
I've been trying to distract myself/prepare for the update on the map that has my cloudlevel highway, and the under-construction world-tree, complete with hole down to... well, a lava pool right over bedrock. Quite good luck for the lava to be dead center in the hole, too. The shaft was dug with dynamite when I got bored with mining and had exhausted all my legitimately-gained dynamite, so I just hacked in a few stacks and tore apart a mountain with it, including using it to bore a hole through the earth.
I started building a seafloor dome, but still can't get the aesthetics right, though I've gotten the procedure down pat. Assemble it on the surface with gravel, let it fall to the bottom, then cover that in glass. Airlocks require a bucket to assemble, usually, in order to counteract the flows by placing springs (since water doesn't flow into a spring on the same level).
Also, "The Slip" just sounds retarded. At least the blog post mentioning prospective names said he didn't really care for it either. The suggestion "tesseract" from the comments is better, if