Daniel Griffiths was one of Doom's early custom contributors, though looking in the past it seems as though he tended more toward the side of deathmatch in his level design. Xenomorph is no exception, but the level's size gives it a bit more character on top of the fact that co-op play was factored in. That's not to say that the author gave single player a pass. In fact, you're encouraged to load it up and run around outside with God mode on to watch the light show. Why ruin a perfectly good game of dodgeball with cheat codes, though?
One important note before you get started: Griffiths included a custom sky as an actual .GIF file. If you want to see it, you'll have to insert it into the PWAD, which took me some fiddling about in SLADE3, but once I figured out that I had to convert the graphic and palette, it worked about as well as could be expected. It's not the first time I've seen this sky, either; it looks like lupinx-Kassman grabbed it from a deathmatch compilation, 1BDWANGO, for use in his NEB03. It definitely beats playing the otherwise low-lit level with Doom's bright, default sky. The new backdrop gives XENO11 a primary theme of contrasting gray against dark blue, which you'll see in some of the interior spaces as well, and helps to create a unified atmosphere.
The moment you drop in, you get the full brunt of Griffiths's monster arrangement. There is a ring of cacodemons surrounding the complex on the outskirts of the level. They don't really offer any pressure because there's so much open area to run around in, but they are a palpable threat if ignored as you're liable to catch ball lightning upside the head when you're not watching yourself. If you're wondering how the author can justify throwing so much meat at you in the beginning, you need only check out (what I thought was) a conspicuous structure on the level's northwest border, behind which you'll find a chaingun, plasma rifle, and rocket launcher plus a bunch of rockets. There are some scattered sergeants that might pepper some of your health down, and some imps on ledges, but nothing serious.
Once you get inside, the gameplay is bog standard room clearing, with a few neat bits like the shutters you open on the southwest and a few secret teleporters. About half the complex is immediately available while the western section remains locked behind the yellow key door. The walk down the battlements and the windows do a great job at giving the level a sense of place beyond being bog standard techbase. About the only thing that really sucks is the section of hallway / tunnel that stretches between the northwest / southwest areas. It looks like the author ran out of ideas to spice things up before the command center-type wing that contains the exit.
Anyway, XENO11 is a pretty cool early Doom level, certainly among the more playable ones from the era unless you're one of those players that detests Doom realism in all its forms. I don't know how it plays in deathmatch but if I had to assume I'd guess that all the cramped rooms and tunnels makes for some pretty awkward flow. NOTE:XENO11 was edited for inclusion in the 1994 Tune-Up Community Project as MAP09, "Bug Hunt!". 1994TU is a mixed bag, but anything with Travers Dunne's name attached is worth checking out.
WE'D BETTER GET BACK,
'CAUSE IT'LL BE DARK SOON,
AND THEY MOSTLY COME OUT AT NIGHT.