There is a way to climb the building that cannot be climbed in Assassins Creed 2.

Ezio only enters this place once when Ezio uses Leonardo's flying machine to soar over and into it.

After that, you cant get back in. I know, I've tried. To get back in, use the "walking on air" glitch to run over the top and into it.

First you must purchase the throwing knife special attack. Then go to any one of the large buildings along the side of the unclimbable one in Venice.

Then, equip your throwing knives and run/walk/sprint toward the ledge facing the building and hold down X. Ezio will run off the edge and on the air.

Once this happens you can release all buttons except X, and Ezio will keep moving until you release X, at which point Ezio will fall out of the air.

To reach the building, run at building off the other opposite roof and hold X to srint accross the air at the building. When you hit the building wall you will keep running against it untill you release X to make Ezio fall.

Release X and quickly press B to grab onto a the wall before you get too low to climb back up it.

Once you have a handhold, simply climp up to the top and bam, you're inside the unclimbable building.