As well as being a suite and guest room, it is also an elevator stop. It is above the Main Lobby and 2nd Floor Lobby and below the Sun Deck. It's the only guest room to be connected to an elevator stop.
Over the goals in the game, three main guests will stay in the Penthouse and act as an antagonist. These are Frankie Fusilli, Ava Cadavra and Optimum Alfred. The Penthouse is only unlocked when Frankie Fusilli arrives, and remains locked from redecorating until the defeat of Optimum Alfred at the end of the game. At this point, the furniture can be moved or picked up and new furniture can be placed.
When Ava Cadavra leaves the hotel, she sets the Penthouse on fire on her way out.
During Optimum Alfred's stay, the player's sim is locked out of the Penthouse by him, and you must collect 5 mechanical skillpoints in order to gain entry. This is the only time the Penthouse isn't accessible after the first antagonist arrives, as it acts as a base for all three antagonists and therefore is a vital part of the game.