From the Super Mario Wiki
World 1 is the first world of Super Mario 3D World. It is a small grassy island with blocky bushes and flowers that lies southwest of World 2 and World 3, south of World 4, east of World 5, and below World 6, World Castle, and World Bowser. ? Blocks and Coins are scattered throughout the map. It contains five standard courses, an Enemy Blockade, and two Toad Houses; one normal house is north of the Enemy Blockade, and the other hidden is in a large shrub in the back of the island, to the left of Bowser's Castle. It is hidden inside the black opening. The level also includes the first Slot Machine, Captain Toad course, and Castle. The boss of the world is Bowser in his Bowser Mobile.
After completing the castle, the Green Sprixie Princess will create a Clear Pipe to World 2. In World 5, there is a green Warp Pipe that can be created below the castle; the pipe takes players to a small island, where the lower green warp pipe will warp the player back to World 1, where it will remain open next to the castle.
After the game is completed, the Sprixie Princesses will build a rocket at the beginning of the island that leads to World Star.