On the horde side you can jump up the side of the tower between the little fence, there is a snow mound next to the hut before the towers and behind near the gy there is a hard to do, but possible jump from a mountain on to the gy.
Ally side of the map there is next to the south tower a little pixel you can jump up, alittle closer to the flag there is a small ramp like snow mound you can jump on a certain pixel half way up and jump again or even slow fall down from a higher platform.
Now I get what you are thinking, if there is a dome around the base how can we finish the bg.
Well you are in luck, if you can’t finish the bg then you don’t need to reque.