I think it requires too much more time. It's a nice idea, perhaps something either to be included in a booster pack or in a DoF fan mod (Dawn of Fantasy Unleased, Dawn of Fantasy Extended - some generic title which includes a ton of fan-made content, which could also have moats as well as saboteurs).
Although I don't think DofE has explosives
Perhaps a Fire spell add-on? Or an Earthen / one for Elves (vines creeping up walls). The 10% idea would fit well with this - the spell could be devastating, or it could have no effect, plus the caster is incapacitated for the duration of the spell - does the attack carry on, hoping that the walls will crumble sooner, or use the spell caster for a better purpose?