The Mermaid Castle is where the mermaids live, it's also located in the Mermaid Lagoon. The castle is described as being beautiful,made from mother of pearl (Also known as Nacre), delicate as a goldfish's skeleton and rises from the bottom of the lagoon to halfway up to the surface. It has no walls except for the wind room, where the mermaids go to rest their gills.
The castle is home to every mermaid in the lagoon. For light,the mermaids use glowing pink seaweed which is held in seashells like vases of flowers or lamps.
A mermaid's room is full of nautical objects which are often used as furniture or decoration. Soop had a table made from a pirate's wooden plank and one mermaid had a bed inside the shell of a clam. The mermaids have whalebone arches. As their are no walls, sea creatures are free to roam in and out of the palace and there isn't much privacy. One room had two crabs running around the bed and many illustrations of the mermaids showed schools of fish swimming through the rooms and starfish in the mermaids' beds. For this reason, every room has a concealment forest. Concealment forests are miniature forests of kelp which shield a mermaid from sight when she wants privacy.
Like the Home Tree, there are many rooms which have other purposes, like the wind room. The castle has a banquet hall where all the mermaids eat. The mermaids' seats are nautilus shells. The shells are biggest at the top of the table where Queen Eewee sits.