The following video shows where is located the Cage Key in Dark Souls and its usage.
The first time you arrive at Sen's Fortress you will find some cages on a side of the first bridge. The Cage Key is a Master Key that can unlock any cage in the Fortress. The best thing is that at the very top of the Fortress you will find out that one of those cages is an elevator.
So first to get the key we start at the bonfire located in the balcony of the Fortress. If you go up the stairs and follow a narrow path you will see a broken bridge. Jump to the other side of the bridge and enter the tower where you will find a merchant. Remove weight for the jump to have more chances of success in the jump. Once in the tower go down and you will find a Man Serpent Mage who is guarding the key.
Once you get the key follow the same path from the bonfire but, this time go the opposite way. You will find the locked cage that is an elevator. Open the cage with the key and use it. It will take you down to the beginning of the area (first bridge). Now you can go through the whole area really quick by using this elevator without risk.
