Crusades & Slayer
Two wads by the same author, Richard Wiles.
Crusades. An episode 4 replacement, using the gothic textures extensively. Highly detailed and pretty much the state of the art for vanilla-compatible Doom mapping in 1999. Most of the maps have a vaguely castle-like brick/wood/metal theme, with blood and gory decorations everywhere - though the secret level is more like a techbase. Interestingly it cannot decide if it wants to be dark, foreboding, and cramped, with a low monster population but even lower resources; or light, airy and abound with carnage. And you end up in Quake.
Gameplay is challenging. Its difficulty level is just on that edge where you keep getting killed but you think "no I should be able to do this" so you keep retrying it. Indeed it was probably the first wad whose on-the-cusp difficulty level caused me to become obsessed with conquering each map from a pistol start. I'm talking hundreds of attempts, and eventual success taking an entire year. The fact that I managed to repeat this achievement recently in less than a week proves I must have improved as player!
In summary, highly recommended. One other thing, it was primarily tested with Doom 95, which is why the intermission screens are only visible on episode 1, whereas the actual maps are on episode 4. This is a known bug with Doom 95 which is fairly easy to fix using a hex editor.
Slayer came out in 2001 and I must admit when I saw it I thought "this is Crusades for Doom 2". However I didn't play it properly until very recently, and there are important differences. Firstly I think the style is more "hell factory" than "hell castle", but what quickly becomes obvious is that it is much harder - of eleven single-player maps I have only managed to do the first and third of them from pistol starts. On most I die very quickly, or frustratingly having an accident after carefully playing the first quarter.
Yes unlike Crusades this is not "on that cusp". More like having to play too defensively (which is itself frustrating) shooting at monsters from far off with weak weapons - can't approach them because it's too dangerous, you'd be in a massive crossfire and there's no health - then desperation sets in as your ammo begins to runs out, and even if you do get lucky the next bit is even worse.
I do recommend you play Slayer though, it has excellent architecture, texturing and so forth, and is good to play if you use savegames and carry your guns and ammunition forward.