I played the original some years ago and now I bought the Serious Sam Humble Bundle games and I've said to myself :let's try the HD reboot. The core of the game is the FPS feeling. You shoot all sorts of monsters: skeletons (the nastiest ones), bulls (horrifying) and some pretty nasty bosses (especially the last one). There are some puzzles (nothing difficult), but the main feature of this game is: THE SHOOTING! Shoot the hordes which advance towards and soon overwhelm you. You feel frustrated at times because of that. You have so little ammo and you quickly die.
Overall a game for any FPS lover.