You can use the door. I can't believe you haven't figured it out with all that practice! All you have to do is ascend the north stairs and break the Fool's Facade to find a chest. Open the ancient door a little to the south and defeat the Brainpan inside. You will find another Brainpan at the top of the stairs to the south. Now climb the stairs to the east, ignore the red guy and kill the final Brainpans. There you go, you entered the building.

Nice.