Kingdom Builder is a featured mode on the mobile game, Super Mario Run. In this mode, players can customize their own kingdom with different objects and decorations. Players can also get Toads to join their kingdom through the game's Toad Rally mode.
When Princess Peach is rescued, or as you make your way through level progressions of the Kingdom Builder Mode, you'll notice that Princess Peach's Castle changes per level. The following screen caps show how Princess Peach's Castle has evolved per level. Every 200 Toads that come to your castle, it levels up.