Inspiration!!!
As I was thinking about this topic some, I started to realize on of the key issues with DF as it is right now. The whole idea of DF, at least for people like me, is the story that you build before your fortress crumbles to ruin, as there is really no other objective. The issue with this is that it is far too easy to make one's fortress impenetrable, and thus avoid all of the loss and tragedy that makes every story good. Currently, if you want this to be part of your story, you have to intentionally seek out the most inhospitable places in your world, or intentionally leave yourself vulnerable. While there is nothing intrinsically wrong with this, it reduces the characteristics that are available to you for the making of a compelling story. No one is going to be very excited about a fort built in the middle of a good forest with low savagery where every siege is destroyed by traps and the clowns only ever get to play checkers.
So how do we find a way to reduce, if not eliminate the impenetrability of a fortress? I think that the answer lies in the basic idea of this post along with the possibility of digging enemies in the future. Not only should every tile, along with constructed walls and bridges, be vulnerable to enemy sappers, the time it takes to destroy a wall should be based on weather or not it is constructed and its composition. In addition, the time required for mining stone should be greatly increased, which would both balance the strength of enemy sappers and increase the importance of miners. And what better thing to do than to make mining a more labor intensive task. Not only are dwarfs a mining oriented race, but it makes all those migrants a bit more useful.
I a good bit of this post is off topic, but the main idea is a salient point, as it would contribute to the usefulness of quicklime as a reagent for making walls stronger.