Climbing around and over buildings is a huge part of the gameplay of all of the Assassin's Creed games. Each city that is used as setting is meant to have a distinct atmosphere that is as historically accurate as possible.

 

“Ubisoft has established something of a reputation in its games for capturing accurately the atmosphere of different historic periods and places, and for Assassin’s Creed III, their creative directors consulted historians, maps, and other resources, to build extraordinarily-exact replicas.”

Dr. Nathaniel Sheidley, article to the Bostonian Society of Boston, MA

 

Above is a painting of the Boston harbor called Vue de Boston vers le Café du Port by Franz Habermann from 1778 (right) and an in-game screenshot of the harbor from Assassin's Creed III. The colors and style of the buildings are very accurate in the recreation.