RTF is a global modification that replaces all towns in the game.

This ability will be useless :(
Because no sane person would send level 1 creatures onto mines/spikes, etc.

It would be better if it were an ability to dig a tunnel 4 squares forward, and then move the usual distance (depending on speed). This would be more suitable.

I don't think that's possible, but we could make them flyers and give them an effect that makes it look like they're digging downwards.