Despite a Keeper's best efforts, dungeons may have some points that remain vulnerable to an opponent's Bomb Wall spell. There is, however, a particularly effective defense against that spell's destructive power: the Reinforced Wall.
It might not be the flashiest of additions to a Keeper's dungeon... but it can serve as quite the nasty surprise to an attacker who decides to throw their Spell-flinging weight around your dungeon. The Reinforced Wall is the only known defense, used to make a wall impenetrable to the Bomb Wall spell, further bolstering any would-be points in a Dungeon's wall-integrity.
Only able to be placed atop existing walls (1x1 per Reinforced Wall), it requires your Workshop to be leveled up to at least level 3, with additional Reinforced Walls becoming available at higher levels of the Workshop.
There are no "charges" associated with this defense; once placed, it lasts forever. Just like my love and affection for you, Keepers!
Ok, fine, maybe that's not the best example... but these Reinforced Walls last a really long time! Consider placing a few strategically throughout your Dungeon, especially if you find that you're having trouble with opponents knocking your normal walls down.