You chose to use the potions as your protection on you quest through the castle. You walk through the door at the other end of the room since that was your only choice other than sitting in the room waiting to die. You walk through the door and see a spiraling stair case. You carefully walk down the stairs all the way to the bottom. At the bottom there is a door. (And since I'm not the kind of guy who is going to play the game for you I'll make a link to go through the door.)
You have one choice you can go through the door.
Choose wisely.