After the initial 'shock and awe' effect, the game seems like an endlessly repeating scene, consisting of:
1: You enter the level and get ambushed by a few generic monters, usually those fireball-tossing things.
2: You run through a bunch of identical fucking corridors fighting identical Generic Slow-Ass Flesh-eating Zombies©, Generic Fucking Camping Counterterrorist Zombies© and Not-So-Unexpected Fireball-Tosser Ambushes© until you find a Perfectly Normal Locked Door©.
3: You backtrack through the Generic Corridors© and find the Conveniently Placed Key©.
4: You back-backtrack to the Perfectly Normal Locked Door© and open it, probably finding:
5: An Incredibly Irreplacable Object Of Extreme Plot Significance©.
6: Rinse and repeat.