Which one is the Abandoned Flat again? Is that the one in Balmora?
Abandoned flat is a little cottage outside of vivec. When you get off the silt strider, you just cross the little bridge near by. Walk up the road to the top of the hill and there it is. Once you have it, go down into the bedroom, pick up the pewter candlestick, and then pick up the key. Walk over to the firefern planter and activate it. It will slide out of the way revealing a trap door to the ultimate hideout with lots of cool shit, including a training facility complete with inactive steam centurions that you can choose their difficulty and activate them to practice combat.
QUOTE(darth_turtur @ Aug 11 2005, 10:37 PM)
Try the Caldera Manor too. It's a three-level house that's got a quazi-Mournhold feel to it. I highly recommend it.