I got all the achievements, completed with all characters so if anyone's got trouble write.... But in the comment box under the game please, i don't watch this forum often.
The game is stricktly linear, going left to right, occasionaly through doors, so you can't miss anything. IF you pass always to the right, you will at a point always reach the end of the area and return to the map.
well, now i see my issue- i play an assassin. there is a wall there, after you kill - that you need to climb somehow. but as far as i know... you can climb or jump from wall to wall in this game.