Is there a way to go wrong when an add-on is just recycling an already great content?
Like the original game, in Death Kings of the Dark Citadel you start the game by choosing one of three available classes: the Fighter, fast and strong; the Mage, a powerful spell caster; and the Cleric, which mixes muscle and magic for a more versatile option. There isn't really a bad choice aside from the fact that you'll find the Cleric a bit weak during your first moments in the campaign. The Fighter will crush the enemies quickly; however you'll have to be a little defter due to the constant exposure during the melee combats. Finally, the Mage is by far the best choice if you aren't a hardcore player, because his first weapon is ranged and unlimited, even though the magician doesn't deal too much damage until you get the real powerful stuff.
Talking about the arsenal, you still have only four weapons per character in Dark Citadel. The fourth one is divided in three pieces which you'll have to find in the dungeons you explore. The good news is that you have much more mana spread in the maps than HeXen ever had, so you can spend as much as you want and still have a huge energy reserve to use your weapons even more.
That's particularly important because this expansion boosts the difficulty too much. There're less weak monsters and more of the tough ones, so you'll find yourself dodging their projectiles and depleting your mana reserve quite often during each struggle. The combat sequences are usually lengthy, and eventually enjoyable if you liked the brutal gameplay of the original HeXen. Most enemies don't die with a single hit, so you'll repeatedly bash one monster until it goes down.
However your challenge isn't only the creepy bestiary present in the mazes of Dark Citadel. In this add-on you still have to solve puzzles and pay attention to hidden switches and doors. For instance, if you forget to activate a particular button, you'll keep going in the levels until you become stuck in a door or something; it's sometimes quite hard to track your way back to push or activate something. That's not only because the maps are literally labyrinths with tiny corridors, but also due to the fact that you'll have to travel between different levels where monsters usually respawn (and there is no reward on killing them, except to clear your path again).
But that's precisely what old-schooling is made of. The classic formula used in games such as Duke Nukem 3D, Shadow Caster and DOOM is reused in Death Kings of the Dark Citadel nicely. You'll be accessing the automap often to find a way out, and you'll have to remember where you first spotted a locked door now that you have the proper key to open it. If this isn't your thing then don't believe that this expansion will satisfy you, because it follows the old-school formula of HeXen faithfully.
The final encounter of Dark Citadel isn't any harder than its original game, but if you play in the hardest difficulty level you'll find that the minions of the final bosses are quite punishing. Luckily this add-on rewards the player with all the necessary artifacts to help in the battles. The Icon of Defender, an item that makes you invincible for a few moments, appears more often than ever in Dark of Citadel. You'll also find a greater use to summon the Dark Servant or to keep your Armor Class high with the Dragonskin Bracers, so you can absorb more damage when surrounded by ranged enemies. Additionally, the Boots of Speed is much more useful now, since dodging powerful projectiles is essential especially when fighting against specters in the cemetery part.
The new campaign takes more than 10 hours to beat, which is enough for an expansion. If you consider that you can replay the game with the other two available characters in a higher difficulty or, even better, in multiplayer up to four players in cooperative mode, you'll find that Dark Citadel is far from being a thin add-on. It would be nice if Raven Software decided to put new weapons and enemies, but it's a very good expansion in the way it is.