I hardly remember much of my first playthrough of either CEZ act, besides giddy laughter at how great it looked and how cool it felt to traverse act 1's greener parts as Sonic. It definitely didn't feel like the kind of level that was meant to wow a first play; it felt more like a mid-game gauntlet, the first real indication of SRB2's later difficulty.
What I have found is that *repeated* playthroughs of Castle Eggman have been immensely satisfying. When you can navigate its traps, you get to focus more on the overall structure, I guess. Where each "room" starts and stops, where paths meet up. And the zone is really complex! The CEZ1 path where you follow some springs up a hill, into the trees, only to then spring back across the entire massive "room" to the other side? That really got me. That got exactly what I wanted out of a level. The skyboxes are beautiful! The colour scheme and the textures! And the challenges, how I love the challenges now! Castle Eggman taught me to no longer be afraid of the swinging spike balls! I've *always* been wary around those in SRB2! Now I'm running through them, intuiting their patterns at a series of faraway glances!
Oh but the greatest part, at least for me, is Castle Eggman's most defining feature: The sense of size. The Firelink bonfire wasn't the only Dark Souls allusion; Eggman must have learned architecture at Anor Londo, because who the heck needs walls to be spaced that far apart all the time, ceilings to be that tall? Better question: what effect does that sheer scale have on players navigating the castle? It makes us uncomfortable in specific ways, and it offers a chance to focus on mastering the game's combat mechanics. ...like Anor Londo in Dark Souls. (And.. it's a mid-game challenging bottleneck. Like Anor Londo.) Like, combat is a much bigger mechanic in CEZ than in any of the previous zones, and I doubt it's an accident. You can also very easily run past everything, again because of the size of the rooms, but when you do stop to fight a Facestabber, or when a Facestabber catches you off guard, you will get some good practice in with circle-strafing, and thus Castle Eggman facillitates that the player is better equipped to enjoy the game in the long-term.
It may be an odd one, but it's also trying a lot of really cool things. These two things can produce *great* stages.