There are signs on the map, similar to the mouse in the game. That's an ancient tomb. Press shift when you get to the edge of that thing. By the way, you just don't know how to get there. The entrance to an ancient tomb is usually in a hidden place. Look at the signs on the map. If you are on a plane, you can't find the entrance. There are only two ways to get clothes in Assassin's Creed 1. One is to spend money, and the other is to invite people with files.