Two hours later we arrived ***to*** the top.

They're both right. Exactly how you have it written is fine, including the "at." If it's a mountain, you can even say "Two hours later, we arrived at the summit" The difference between "two hours later" and "after two hours" is subtle. If you use either it serves the same function, but there may be a subtlety that each one carries. For instance, "two hours later" sounds like you're telling a story and a little more factual. "after two hours" sounds like you're putting emphasis on the length of time. You can also say, "After two hours of climbing, we arrived at the top." With that example, I as a reader imagine the two hours of walking. "Two hours later, we arrived at the top." For me, in my mind, this happens instantaneously. You just went from point A to point B in my mind without all the hardship of climbing. So it depends on what effect you want to go for. Either is fine. Lastly, you can kind of combine both if you want. "Two hours later, after climbing through treacherous terrain, we arrived at the top." The second clause isn't of time. But let's say you wanted to force that imagine into the readers head, that can come after and serve that function.