But the "combat classes" are only slightly more effective than an army of miners. If you try stopping them from mining, they'll just block off the passage with stone. If you attack them when they're together, they all just gang up on you. If you try shooting them with arrows, they simply build something and hide behind it. If you try attacking their vault, it's inside a huge mess of stone and doors right next to their spawn point, and they just attack you as soon as they spawn. If they want defenses, they can build catapults and large defensive positions. It's messed up, man!
It's kind of like playing Rock, Paper, Death Ray. Sure, a Rock or piece of paper can stop a death ray if he doesn't know what he's doing, but if he does, he'll zap you 70% of the time, every time. The only time builders are vulnerable is if they make a mistake, are in the open, or don't have any stone. So, really, at least one class should be able to destroy fortifications easily (That's not a builder). Bombs don't seem to cut it.
I've only managed to get to the enemy's gold once, and that was in my first game (Surprisingly). Stealing is obviously supposed to be a big part of the game, but it's so hard right now. And digging for gold is pretty boring.
It's still a fun game if you get a nice mix going, though. Archer's probably my favorite class.
Also, solution to your stone problem - Go kill enemy builders. I've found they usually spend a lot of time getting lots of stone.