On cs_assault, you can get on top of a building by the CT Spawn.
Go up the stairs to the top of the train station.
On the outer part of the level there is a fence across a wall. There is a tiny ledge under the fence. Jump on that ledge and start walking across.
when you run into a fence post you will stop. To get around the post, either slowly walk around it, or just carefully jump around it.
Once you are on the building, there's not much you can do except surprise the enemy if they happen to come in your range.
