First destroy all Battle Droids, once done make your way to the enrty hall where you started, all the doors should be green. Go forward once, up once, left twice, down twice and left twice. You should now be at the bottom left corner. Doors should turn a yellow color now. Go right to the very end and the doors should turn red (I believe). Now go left once, up twice, right once, you should be at the upper right corner near enterance and all the doors should be a purple color. Now go left twice, down twice, left twice and the door should be open.
Mind you, this is the method for the Dreamcast version, not PSX. If you need more help, check out my FAQ on www.gamefaqs.com under Dreamcast.
You know, sometimes I even manage to amaze myself.