zrrion built up a nice little portfolio in 2012, paving the way toward his eventual inclusion as one of the guiding forces behind the ZDoom Community Map Project Take Two. After exploding with a big ol' Realm 667 showcase in The Void of Hell, he followed up with a more muted showing in The Blood of Virgins, which used GZDoom to enhance what would otherwise be - I think - a traditional experience. The Core is, then, a more substantial experience, a larger level with about three times as many monsters to chew through. Like its predecessors, it was released in 2012 and targeted GZDoom. The pacing and layout leave it with a feeling more akin to adventure maps, though, than anything that has come before.
A_CORE doesn't have a plot, except to say that Doomguy has arrived at some sort of techbase and just knows deep down that something isn't right. You'll find snatches of cryptic quotations during normal progression and from some of the secrets, but I have yet to see Fight Club, which is where they all come from. I guess zrrion likes Fight Club! The semi-secret red skull key and its corresponding switch in the outpost of Hell was a long way to go to get a berserk pack, I think, but sometimes the journey is its own reward, especially when you can stumble upon what I believe is a shoutout to fellow ZDoom user WildWeasel if you're willing to do some mountain goating. And I think that's where zrrion likes to leave the realm of stuff accessible by jumping - inessential secrets, with the main progression all doable with strolling. Which is an unusual, but welcome, compromise.
There aren't a whole lot of gameplay changes beyond one important detail. zrrion has changed up the HUD, and with it, included a sort of regeneration system that's a step above vanilla health mechanics. Essentially, any health packs you pick up - and they'll look very different from what you're used to - go into a separate pool which then refills your health at a moderate pace whenever you take damage. I think that it's a pretty cool tweak since it keeps you from scrounging for health, which would otherwise bust up the pacing, and while it won't save you from a direct Cyberdemon rocket hit, it'll help you out if the demons get one in over on you and you had a nice buffer built up from some healthy caution.
As far as the setting goes, I think I like zrrion's craft more when he's not trying to do man-made (or demon-made) stuff, which is almost unilaterally filigreed to death. The naturalistic landscapes and huge setpieces like the dam are more palatable, I guess because the topography of outdoor areas can only be iterated upon so much without looking ridiculous, unlike the network of lovingly and repetitively detailed corridors that makes up much of the base portion. You can make some pretty interesting rooms and architecture in the original Doom engine and that potential only gets better once you escape vanilla limitations. The base portions of The Core have flashes of this here and there, but no amount of detailing will jazz up those corridors.
But I really like the little jaunt to Hell (from an aesthetic standpoint) and the ways it intrudes upon the dimension that the base occupies, like the grated corridor that explodes into a good ol' fashioned firefight, one of the most engrossing encounters of the entire level. Contrast against the battle to leave Hell, which pits you against a low-energy series of ambushes starting with four zombies, then four imps, four demons, and finally four cacodemons. I found the much-expected behind-the-back revenant teleport ambush at the plasma rifle pickup a much more dynamic element. The rest of the level's combat is, much in keeping with its adventure-style, "what you see is what you get" corridor and room clearing with the occasional monster closet-style trap. Several portions vary up the height and turn the action into sort of a canyon crawl, particularly the final leg of your journey, past the dam.
Among some of the cooler set pieces is some kind of weird flesh tunnel off of the base that lowers a teleporter that grants access to a higher tier that you likely glimpsed in an earlier encounter. That room, too, is kind of cool, with the back half overtaken by some kind of FIREBLU field that marks the edge of demonic corruption. The actual fight is... not as interesting, especially when the lift pit in the center appears to be some kind of holding pen for a Baron of Hell, but that falls more into the realm of world-building and works pretty well in the sort of slow-burn context of an adventure level. I also kind of like the little library wing.
And, well, I'm pretty sure that zrrion got away from a lot of these criticisms with his contributions to ZDCMP2. It'll be cool to see what he did some three years after this, though, with 2015's Mouldy Old Void. If you like some moderate action with an adventurous spirit and don't mind detailing that borders on the baroque, then you might want to kick around in zrrion's Core.