The three maps are Xen caves and passageways, populated with houndeyes, headcrabs, slaves and zombies. The architecture is basic, but sound - the houndeyes, however, are all facing the wrong way and are inactive as a result so it's just a case of wandering through killing them from behind for the few minutes that each map takes to complete.
This makes it pretty unsatisfying, which - sadly, sums up the three maps. Which is a shame, as with some monster tweaking, and maybe a few traps or something, they could actually be quite fun..