Once you're at the fortress, enter through the guards' entrance. Then, push the hidden door, which is located on the wall that's blocking you from a ladder. Now, climb up the first ladder, and then climb up again on the next ladder. Now, there will be a few boxes blocking your way, so you'll have to go east and you should find a ladder leading downwards. Climb down. Next, climb up the ladder in a room to the east of you. Now you'll be on the other side of the boxes that were blocking you before.

Now, you should be in a room with an altar. You may recharge your prayer and collect yourself here if you are low on hitpoints or prayer points. After you've rested, continue through the door to the west. Run past the Black Knights and go down a ladder. Finally, you'll be in a narrow hallway. Go to the grill which is on the wall of the hallway and choose "Listen-at Grill". It will go to a cut-scene of some evil characters talking about an invincibility potion. Your job is to stop them.