When you enter BRS, you will be in an "entry room" from which you can enter the Upper Spire (on the left) or the Lower Spire (on the right). The Upper Spire requires the Seal of Ascension, and is entered through the door on left just up the short stairs. The Lower Spire can be reached by simply jumping off a ledge through the archway on the right-hand side, just up the stairs. After entering the first room, pull the 2 orcs on the left. Kill the patrols, and move to the bridge. Jump down the drop-off on the right side of the bridge.