Rex Claussen started out under the moniker of Gurkha Boy, making levels for limit-removing ports after enjoying Ian Wilson's Herian 2 as well as some correspondence. Arena, his fifth release, breaks with the source-port requirement for the first time for an experience that can be enjoyed in vanilla Doom II. It's a MAP22 replacement released in 1999, and it's also really, really easy to explain because it's actually a scant 19 monsters deep. It's literally an arena, of course, and nothing more. One of those interstitial ambushes as Doomguy travels between larger adventures, I suppose.
The setup is rudimentary but effective... and similar to the lead-off fight of MAP03 from his earlier Quo Vadis. You start out staring down a Cyberdemon and no way out except for all the caged switches on the periphery. One of them ought to stand out, and it's shootable, which while opening up the first fully barred alcove also wakes up the reinforcements. You'll also be able to see your weapons of choice in elevated cubby holes. The level's action involves dodging Cyberdemon rockets and flipping switches, each one opening up another alcove that will be difficult to spot because of the low, flashing lighting and ushering in a few monsters that might catch a glancing blow but will more than likely distract ol' Cybie.
This action continues to the tune of about 17 mid-tier monsters teleporting into the arena, the last switch opening up the exit and lowering your weapon and ammo as well as unleashing a second Cyberdemon. You can either cut and run or try to finish out with a long and slightly thrilling but not really waltz. It's the kind of encounter that seems fit for a level's finale but looks proof-of-concept here, presumably the only fight in the level because it depends on the player not having enough ammo to just grease the Cyberdemon before starting the festivities. If you're not looking for anything deeper than a little warm-up exercise, then Arena might well fit the bill. Otherwise, well, there's plenty of other things out there to sink your teeth into.