Valley of Bowser
From the Super Mario Wiki
The Valley of Bowser is the final world of Super Mario World before reaching the final boss, Bowser. The main way into the Valley of Bowser is by beating the Sunken Ghost Ship and entering the massive rock carving of Bowser's head that rises out of the sea. A faster way to enter is using the Star Road, which has an exit leading straight to the Front Door of Bowser's Castle, making it possible to beat the game while skipping almost every world in the game. The player can also see this area by using the warp pipe found in Donut Plains to access Donut Secret 2 (the level upon the cliff on the left). Additionally, the player can enter the valley using the secret warp pipe on Chocolate Island, which leads to Chocolate Secret (the level at the bottom of the map).
There are frequent flashes of lightning in the background despite this level being accessed via a subterranean cavern. Various enemies, though mostly Koopa Troopas and cave-dwelling enemies, can be encountered here. What is presumably the Valley of Bowser, renamed the Valley of the Koopas, appears in the Nintendo Adventure Books Dinosaur Dilemma and Flown the Koopa. Depicted as a barren, putrid, desert-adjacent wasteland filled with garbage and hostile creatures such as Munchers and Dino-Torches, the area contains Bowser's summer home, where he holds a party celebrating his granting of the Snowbell Prize in Dinosaur Dilemma.
Note that levels are listed in the order the player can access them. Also, levels with an asterisk (*) feature more than one exit.
There is an unused Red Switch Palace to the left of Valley of Bowser 3. Since it leads to an "empty" level, it is probable that it was scrapped early in development. Valley of Bowser 3 connects to its path, and unlike other levels with just one exit, Valley of Bowser 3 is set to move Mario to the left when its secret exit is activated instead of up. Presumably, this means that Valley of Bowser 3 was originally going to have a secret exit at one point before it, too, got scrapped. If a secret exit is hacked in, the game glitches, activating Valley of Bowser 4's normal exit and trapping Mario in such a way that he would be locked in an endless Bonus Game.
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