When Mario and Luigi first visit the castle, the frames on the wall are empty. As levels are scanned through an e-Reader, each one containing an e-Coin has the coin's frame indented and its level number in the plaque below (the game's Wii U Virtual Console and Game Boy Advance - Nintendo Switch Online releases have the frames and plaques marked from the outset since the e-Reader levels are built in). As such, the castle appears to be a sort of museum or trophy gallery.
The castle has three floors of eight frames each, with the second and third floors (2F and 3F) opening to Mario and Luigi once the player scans a level containing a 9th and 17th e-Coin. The player can then teleport between floors by pressing and . However, this functionality goes unused because only eight e-Coin levels (Super Leaf, Starman, Toad, Mario, Princess Peach, Luigi, Mushroom, Fire Flower) were ever released, allowing for the completion of 1F. Prior to the Virtual Console and Switch Online releases, due to the full spread of e-Reader levels not being released outside Japan, American and Australian players were able to collect only three e-Coins (Super Leaf, Starman, Mushroom).
Modifying the game's code to have 24 e-Coins collected, thereby filling all of the castle's frames, does nothing.
- The music played in the castle is very similar to the theme of Mushroom Kingdom in Super Mario RPG: Legend of the Seven Stars.
- Hodgson, David S J. Super Mario Advance 4: Super Mario Bros. 3 Prima's Official Strategy Guide. Page 113.