Mario was speeding past through all of the worlds. The Koopas couldn't stop him so we built a Tower halfway through. In the first game the only worlds with the Towers were W7, W6, W5 and W4. World 8 had two castles. In the second game all the worlds had Towers. In W5 there was two towers. In the third game all of the world's are "normal" and have 1 tower and 1 castle.

-Bowser Jr.