The Level 50 Tower is Briskbreeze, and it's on Colossus Boulevard, across the street from Kirby Longspear.
There's a tower in Marleybone that doesn't require Level 50 (just that you finished Malistaire's instance); it's at the lower right corner of your map. You have to do a few other things first.
The Tower in Marleybone doesn't require Grandmaster status; it only requires that the wizard finished "The Final Countdown" (Malistaire). That tower can be found at the southeast corner of the map.
