You need to disable the generators in order to get to Barnabus. Here is a
Youtube video of someone doing that part in D2:ED, starting at 3 minutes in for the platform section.
As Lucreel said, there is no way to disable the anti-dragon shields around the harbour or High Hall, and no way to enter Rivertown (the only thing in Rivertown Gorge is the flying fortress).
Try the ministry again at a higher level, and/or when you have better equipment and can do damage faster.
In D2:ED I stood a little in front of Zandalor to cover the two front rifts, occasionally having to Rush Attack a memory eater from the rift in the back. I attacked opponents from the front rifts with a bow first, which didn't do a great deal of damage (didn't invest in any ranger skills), but it did help draw opponents to me, rather than have them go directly to Zandalor. My creature (with a mage head) did a pretty good job of taking care of creatures from the left rift. A demon summon (etc) should help, as well, if you have the skill or the points to try it.
I was level 35 or 36 when I did that section, one of the final things before heading to the Hall of Echoes (duel wielding maxed, level 10 Bleed, etc).
