First off, let me disclaim that I have never been to Silent Tower in any capacity. To be honest, it makes me pretty bummed that I'm going to miss out entirely on what was there in the first place because of a few bad apples. I can only imagine how you feel about it.

Second, I think its actually important to have the kind of explore area you can very slowly progress through on your own if you're smart. Not everyone likes the massive group method, and I think you're one of those people- like me. Perhaps the solution lies in not making it more like the dragon lairs and hell, but more unique, more solo-adventure-y(tm). The down side to this of course, is that its more work that you would have to do, and still has the vulnerability which closed the area in the first place.

Another idea: Change the area so that every class essentially gets a different area. Yeah, I know I'm good at ideas what take a lot of work. Questy class-specific entrances (probably through the same mob, but different), which leads into a small puzzle riddled area with whatever else you want to stick in it. I have all sorts of ideas for nifty things, rewards, traps that could be stuck in for a bunch of the classes that would be best to e-mail about if this idea is viable. If its not, well, its a lot of work and I love to theorycraft as they say.