The Fiendlord's Keep (also known as Magus' Castle in the SNES/PS version) is a location in Chrono Trigger that appears in 600 AD. It is Magus' castle and, according to legend, is home to one hundred monsters.
Being the heart of the fiend world, the keep is permeated with magic. At first, there are two wings accessible. They can be completed in any order. The West Wing is populated with monsters who use mostly physical attacks, leading up to the battle with Slash. while The East Wing is populated with monsters who use a great deal of magic, ending in the encounter with Flea.
After both wings have been beaten, returning to the main hall will reveal a secret entrance to a third, central wing, hidden behind an illusion. This third wing is filled with boobytraps from Ozzie's devising. After passing all of his boobytraps, you will come face-to-face with the general. Defeating him is trivial, as you can use his boobytraps against him and drop him into the basement through a trap door. This opens the way to a stair that descends into the catacombs beneath the keep, leading to Magus' sanctum.
Enemies usually attack in groups. These groups start small at the beginning of long halls and gradually get larger and stronger. Often, these groups are set up in such a way that there is a trap if you fight them in the wrong order. For a good example, Outlaws are often seen with Jugglers. If you attack an Outlaw when the Juggler is still alive, they perform a devastating Fire Whirl Dual-Tech counterattack. If the Juggler is destroyed, the Outlaw will not counterattack at all.
The Fiendlord's Keep makes an Cameo appearance in Final Fantasy XIII as an area with a castle of the Nautilus Park where Sazh Katzroy alongside Oerba Dia Vanille fights Sazh's Eidolon, Brynhildr. Additionally, Brynhildr wields a scythe-like bludgeon when in non-Gestalt form, much like the Keep's owner from Chrono Trigger, Magus.