I think there's a little gamism vs reality thing about wall destroyers/sappers/tunnelers that makes them a little weird.
In reality you might not know you actually have guys busting in one of your walls someplace - at least, not immediately like games do. I regrettably haven't checked this game in particular out yet, sadly, so I dunno how you deal with it specifically, but like, when invaders come, you get the little notification, and then even if you can't see them breaking stuff, you can probably see the world tiles getting broken, so you know exactly where it's happening. The only way you would know in real life that dudes were tunneling or whatever is if sentries or someone saw them and reported damage or weird people in an area, or maybe if you heard them working.
I guess I'm just babbling out a tangent or something, but I guess I'm just thinking it would be cool if you had a fog of war, or maybe even a total blackout of parts of the map your units couldn't see. That way, you'd have to send out patrols or station guards/guardtowers/whatever, and if you didn't have good enough coverage enemies might slip through. You might also have groups suddenly happen on evidence of, say, enemy raids or damage or whatever. Would also make some meta-game stuff like 'how many civvie guys do I have/need and can they support this many military guys' and 'how thin do I spread patrols' and stuff I guess.
Though, I guess it's a moot thing to talk about, since DF sorta already does this, ever since vampires and stuff got in. I haven't played it in a long while aaa.