Some of the images in Catechumen are stunning. Most are very good. The textures are all very well done. In some cases you'll find yourself staring at the graphics simply for the pleasure of looking at them. The game crashed only once on me and that was because Windows decided I needed more disk space and figured my game could wait on its more important task.
Level design varies from adequate to very good.
I don't know that I felt any one level was superb though I often found myself very impressed by certain sections of most levels. Some of the levels were designed like mazes and others designed with buttons or switches which open other doors. All in all, there was a wide variety. Repetition was never a problem. The system requirements are quite high, though any higher end Pentium should run the game reasonably well.