The ledge behind the bases-
Just bring a few plasmas and frags (although, I recommend bringing an FRG or a rocket launcher). FRG or nade yourself onto the top of the teleporter behind the base. Then plasma, frag, and rocket/FRG (you don't have to, but it's a lot easier using it). Run into the os as you throw the frag and FRG/rocket and jump. It may take a few tries, but you should get up there. You'll get a pretty good view of the base. I highly recommend this spot if you're camping the other team's base.
You could also get on the high rock in front of the bases, but you'll need a vehicle on the little snow-covered rock and maybe FRG/nade. I've done it a few times, but it's very difficult.
Also, on those rocks, there are some small ledges near the opposite end of it that you can get up on with a hog and FRG. Not a great spot, but pretty ideal for base protection.
There are a few ledges by the bridge in the middle you can use.
There's one on the lighter side of the bridge that you can get up on without nades or any heavies. Just jump on the little slanted ledge on the bridge and jump against it.
On the dark side by the sniper spawn/teleporter, there's another good ledge. You can FRG yourself up. You're also by the sniper/health spawn, so that helps.
Hope these small tips help you out.