In the area around the entrance to Ulysses's temple, there is a marked men camp. I took out three of them located there. I noticed another hostile mark on my perception and looked up at a cliff above the camp and temple and saw a marked men sniper. I could not get to his body after sniping him, but it looks like there is room up there and a fire barrel too. Has anybody been able to get up on the cliff and if so, how? I do not remember any areas in the game where characters are present that the player could not travel.

I think i recall that spot. Or at least one similar. It looks like they are just isolated sniper spots far as i could tell.

Yeah, if you look at the temple door, it is directly above it. There is also another small cliff to the left looking up when facing the temple door as well. Not sure if any of them can be reached. I just did so much climbing on random objects throughout this dlc that I was hoping there was a way up somehow.

Both those areas have warheads so I think the idea is that you can set them off to take out the enemies, but not access them otherwise.

Nice little note, the enemies will fall to a reachable location once they go boom.