Ridley (also Ridley's Lair) is an area in Norfair.
In the original Metroid, Ridley's Lair (also known as Mini-Boss Hideout II) is made up of several long corridors, and there are a lot of areas where it is easy for Samus to take a one-way drop. Many of the walls are a bright pink color. Common enemies found are Zeebos, Holtz, Violas, Multiviolas, and Dessegeegas. Lava is commonly found in this area like other rooms in Norfair. In order to reach this area, Samus needs Morph Ball and bombs.
Metroid: Zero Mission
In Metroid: Zero Mission, the most notable change is that in order to get into Ridley's Lair, Samus will have to defeat an Imago that is trapped in some vines. Inside Ridley's Lair, most rooms may be accessed without the Varia Suit equipped. Additionally, in order to get to Ridley himself, Samus must obtain one of the Unknown Items. Defeating Ridley does not grant Samus any major power-ups unlike Kraid, but there is an Energy Tank to the left of his boss room just like in the original and in Super Metroid.
While there isn't a Ridley's Lair by title, there is a designated area for him located within Norfair. His hideout is south of the area, and there is a gate in the shape of his head to signify the entrance of his lair. In order to access his lair, Samus must have both the Space Jump and the Gravity Suit to get inside, as it is blocked by a deep pool of lava. Ridley's Lair has a lot of the acidic lava that will damage Samus even with the Gravity Suit on. Additionally, the landscape consists of a lot of ancient ruins. Samus will also have to fight a Golden Torizo to obtain the Screw Attack.