I was wondering if there is a Dwmer ruin that allowed you repeated access to Blackreach? I returned to the Tower of Mzark, and I found out it was a one way trip, same thing with Aftland. I was wondering a Mzinchaleft, it had a door to get there but I had other matters to attend to at the time so I left. Or is there another one that does?

Try and read the following: http://www.uesp.net/wiki/Skyrim:Blackreach

You can open up express elevators directly to the surface, from Blackreach.

Look around for elevator buildings that'll take you directly to 'Skyrim'.

Thanks for the info, much appreciated.