yeah, i think to leave it as is and just make it a mutator is the definitive choice. having the option is always the best. a workaround for stagnant games is to just attack multiple points at once with devices/spells.
1. Imps can't dig through fortified walls, at least under normal circumstances.
2. There are spells and/or devices deeper in the Veins that allow you to get through fortified walls, though these will be costly so they can't just be spammed. They must be used strategically.
3. The Seal ritual fortifies all of your walls and makes them completely invulnerable, even to the things that can get through them, for a period of time.
That sounds decent enough and I like your previous post on it aswell...you would need to periodically check your walls for incoming assaults and cast your spell to save a wall from a breach. A cat and mouse type of game, because once they breach that's it (well you can start putting up doors and more traps etc).
A fortified wall strength bar-o-meter would be good to slide up and down for multiplayer. Some will want weak/normal/strong/invulnerable walls and that would sort that issue out for the community - no one would be able to give out then
you would need to periodically check your walls for incoming assaults and cast your spell to save a wall from a breach. A cat and mouse type of game, because once they breach that's it (well you can start putting up doors and more traps etc).
remember you're going to be able to make replacement walls (not sure if these will be as strong/'unbreakable' as normal walls, will probably be a balance thing)
granted you would have to push back any attacks in order to probably set a replacement wall up in the first place, but a breach won't be the end of it like DK1 if you broke into a high level hero portal and they just kept spewing them out at you ever minute.
to be honest I have never played DK multiplayer so I never suffered from this issue. I have used it in campaign but only to contain enemy creatures in an empty room until I decide what to do with them, instead of using the prison and them turning to skeletons, and it worked just fine.
I am really unaware of any issues it caused in game but it was not broken for me.