I've been thinking, tunnelers would add some much needed ranged siege to Civs that don't have any.  I was thinking tunnel entrances could be built by citizen soldiers, and only when garrisoned fully (10 troops?), it would produce a slow, low, and powerful building-targeting projectile, perhaps with its own ground decal and particle effect. They'd have the same 100 health and 100%pierce armor as siege as well. They'd cost a lot of wood and take a long time to build.  This is just brains