Well, it's been a long time since I've played... but I'm pretty sure I remember how. First, you'll need Erdrick's Armor. This can be found in the ghost town (the name escapes me at the moment) near the western end of the map in the desert. Go to the east side of town and walk near one of the trees (i can't remember which: try em all) until you start battle with an axe knight. beat him, search right where he was standing (or right next to it... damn my memory!) to get the Armor. Now, go buy 6 keys from one of the towns that sells em. That one near the east end of the map has em pretty cheap. Now, head back to the castle. You know those doors you couldn't get past at the beginning of the game? Head through those (don't waste any on side passages. But if you do run out, there's a guy deep in the castle that sells em.) Now, this is where it gets fuzzy. I think you have to walk just outside the walls at the top of the castle, but not go to the world map. walk right up against the wall, following the wall to the right and then down. You should find steps. Walk down these steps to find the Stones of Sunlight. Like I said, that may not be completely right because i haven't played in years, but i think it's right. Good luck!

just get keys and then go through your home castle. You don't need to bother with Edrick's Armor