Lots of different attack types and in the back half they're thrown at you in random groupings which makes for interesting fights.
The enemies themselves are designed very well. The 2 secret levels I found were excellent, so I would assume they're all worth hunting for. There's some good monster traps (especially later in the game) that helps break up the monotony of the constant backtracking. Tight corriders are made tense with good enemy placement while the open areas have enemies attacking you from every direction. The color palette can cause the levels to feel pretty same-y but the level design and overall flow of the game is top notch. I think the game gets off to a slow start but if you can make it past the first hour or so you'll be rewarded.
