Right after you pass the three cyclops with rotating fists in Athens, and you pass the switching green/blue chest, you'll see a ladder. When you climb it, you'll see a dead guy draped over the rafter. Do a double jump over him from the very top of the ladder and you'll land on this thing with a red chest. Then jump to the room on the left to get some free orbs.

Right afterwards, you'll come to a ballista(giant bow/arrow thingy). When you turn the lever it spins around, BUT, after you shoot the door down you can spin it more and shoot through the walls on either side. (the room on the right is something you'll come to later)