Aren't underground castles more of a fantasy thing?
Underground Castle is a single-map (technically, but in practice we've got six maps for you today) scenario for the Second Encounter made by Sean Howard. The map is conceptually unique among Mac mods; you, in fact, start among blood-tainted brown walls and make your way towards a castle-like structure where many adventures and gunfights await. The map is mildly difficult and quite well made. But that's not all! While the scenario ends on map 1, the file also contains two other maps (called "TEST" in ResEdit). The first one is a very simplistic showdown against some Officers, the other one returns to castley themes with what appears to be a tower. Furthermore, there are three other releases, called Underground Castle 1.0, 1.0.1 and 1.5 that each contain the first map of Underground Castle and nothing else. In fact, I have no idea what the version numbers are supposed to represent, since as far as I can tell, there are no differences between the three. Odd stuff, but there you to.
===CREDITS===
Sean Connery.