I made a long post about the climbing in ACII and why the game was pretty good compared to the rest of the series, but I can't search the /v/ archives on fireden for some reason.
ACII not only had tall buildings, but the buildings were very close together which meant you could get anywhere after climbing. Narrow streets also made it more crowded with less NPCs than Unity.
The shoving system was also nice, in II if you sprinted, you would collapse if you bumped into people, and running+shoving was simply slower than sprinting and you couldn't for example catch a running NPC via running or running+shoving.
With the very closely placed buildings, what this effectively created was a fast travel system in the rooftops, where you could freely sprint and this method of traveling would be simply faster than running on the ground.
AC games started to make streets very wide, giving you plenty of room to avoid groups of civilians, and automatically shoving players. Also buildings even in Brotherhood began being far apart, meaning you couldn't climb and then pick a direction, shit felt way more confined to those line connection between rooftop.
All this while navigation/climbing is the feature that sets the franchise apart, in Syndicate they finally admitted they don't understand what the series is about by making extremely wide streets, giving you TWO ways to travel faster than climbing, carriages and grappling hook. It's like giving you button in Doom that allows you to skip combat entirely.