This spot can be accessed from either side. The entrance are however very exposed so make sure you are not followed when you enter.

There are 2 levels: you can stay on the top level to keep a line of sight to the South. You can even take some sniper rifle shots if you want to steal some kills in a fight accross the bridge. On the lower level, you are invisible from the outside and it is quite hard to land good grenades as you can move to the upper level if needed.