To be honest, whilst Castle Marina is a bit nearer to DTD, its still a fair distance and the extra distance to the city centre isn"t that much of a bind but with the bonus that theres lots of stuff to do on your doorstep.
Also, a cheap but okay place to stay which is very central is the Brittania hotel. Recently I stayed there on a work night out on a Friday for £50, and it was fine as a place to get your head down. For the degens out there, it is also next door to one of the city centre casinos....