There should be a way to prevent the diggers from effectively destroying the defenses every time they strike. water walls and magma honeycomb are fine by me but you cant deny that they are quite megaprojects like in what they involve. Bringing the magma up, digging the water wall. Putting it under pressure. All of it seems like a giant finger to the newbies. Yes we can turn it off, but as for now you can enjoy invaders without having to turn them off. I fear overpowered diggers to be able to destroy even most advanced fortress if their overseer hasnt engaged in a form of megaproject like waterwall...
Now just try a thoguht experience : newbiehalls dwarf fortress. Tell me, shall we have to spend 5 years only preparing a pumpstack for a magma moat? How would a newbie perceive that?
Kind of "yeah I managed to get everything up and running, it took me five years but now my dwarves have food and beer. Lets focus on the military...
Aaah crap a tunnel has opened, the fort entrance has caved in, my poorly trained marksdwarves are getting skewered, my defense plans screwed because I could not set up a magma pumping system fast enough!
Maybe I'm not dwarven enough to find it fun but making the game require a mini megaproject from the player for him to see a new feature... Is it an improvment? Those diggas are kind of all or nothing and currently with how maconry works if they pierce your inner walls you are screwed. If we had barricades, we could prepare primary and secundary fall back position, in case all walls are breached but then, why would not they be able to dig straight down ten level and open a tunnel right into the hospital?
I dont like to be a doom sayer but I foresee an overpowered game breaking feature that only most advanced players will be able to deal with.