From the Super Mario Wiki
World Star-Castle is the final castle in New Super Mario Bros. 2.
There are two parts to the castle. The first part has many Grinders and also has the Koopalings blasting Mario or Luigi like they did in Bowser's Castle. There are also ropes and fences Mario or Luigi can climb on. The second is the same but Mario/Luigi are now on a Snake Block instead of ropes and fences. At the end, Mario or Luigi battle Dry Bowser. This battle is the same as the Bowser's Castle level. On the first phase, he will breathe out several blue Fireballs before jumping, allowing Mario or Luigi to hit the ! Switch. On the second phase, the Koopalings make Dry Bowser large. Mario or Luigi will have to jump on some platforms to reach the giant ! Switch and defeat him.
This course is never selected in the Coin Rush game mode, simply due to the length of the Dry Bowser battle.