Tower of Gaan-Telet
Gaan-Talet is a difficult, multi-layered dungeon. Like Halls of Steel, there is a Hard and Vicious difficulty available. Hard is unlocked by collecting at least one of every monster trophy at the Taxidermist. Vicious (often considered to be the most difficult dungeon in the game) is unlocked by completing the quest to defeat Hard with the Purist badge.
The exterior of Gaan-Talet contains the shops, glyphs, and boosters of a normal dungeon. There are no altars (on the outside), and there is no normal subdungeon, although the Smuggler's Den will appear if you prep it. Ordinary monsters from level 1-7 appear on the outside; there are no level 8+ monsters or bosses on the outside. The inside of the tower begins revealed.
To complete Gaan-Talet, you must enter a series of special subdungeons inside the tower. Once all enemies in each level are defeated, the next becomes available.
On Hard, you must clear three subdungeons, plus Horatio's. On Vicious, you must clear 9.
Each subdungeon is a small level, initially unexplored, containing several tough monsters (one of which is exotic), an altar, and a exotic, useful item. On Vicious, this means that every altar will eventually be available. To make life even harder, most levels will give you some kind of penalty when you enter. (A few, including the first, do not.)
The first, second, and third subdungeons contain monsters of level 8, 9, and 10, respectively. The 10th level monsters are not bosses, simply scaled-up monsters (for instance, the 10th level Goo Blob has Physical Resistance of only 50%, not 75%). In Vicious, subdungeons 4-9 also have 10th-level monsters; on each level, the monsters get a progressively larger HP and Attack bonus.
The final boss is the Goat-shaped Horatio the Immortal. Despite his form, he does a relatively modest 75 damage. On Hard, he has 999 HP. On Vicious, he has 5000 HP, Physical and Magical restances of 50%, and Berserks at 50%.
On Vicious, Horatio's subdungeon will afflict you with the remaining penalty, so be alert.