This level is pretty much a Boss Rush. Before you can reach Bowser, you have to go through his castle's sinister passage, facing bosses from previous kingdoms (except for Wart) along the way. The level is divided into several parts.
The first part, the starting point, has an extra life nearby. Other than that, it's not a really long passage with some colored gems to pick up. After you go far enough, you'll face the Happy Tree Kingdom's boss. When you fight it, the only difference in the boss room compared to the original is the lack of Heart items and there's only one floating platform, a block, for you to stand on instead of two larger platforms.
After you've beaten Intruder, the second section, much like the first, also contains an extra life nearby your spawn point. The path also contains colored gems for you to pick up. Considering that the second boss is underwater, you have to swim to reach it. Instead of water, however, you swim through what appears to be blood. During the fight against it, the boss room layout is the same as in the original.
The next section after that also contains a free extra life for you to pick up, as well as colored gems. Once you've walked far enough, you'll meet the Desert Kingdom boss, The Red Dragon. Much like the fight against the previous boss, The Red Dragon's boss room layout is the same as in the original.
The fourth section has two pits filled with lava, but bridges are placed above them to keep you from falling. When you arrive in the next boss room, you'll meet Fire Dragon. The only difference in the boss room's layout is the location of the extra life, which is on the ground unlike in the original.
The fifth section after that has some blocks of ice on the path, signifying that you're going to meet Buirun, boss of the Ice Kingdom. There are also several blue gems along the way. The boss room's layout is basically the same, but seeing that the ground isn't icy, you only need to worry about walking on the icy blocks.
After all of that, you'll be placed on a seemingly empty path where you end up fighting Birdo like in other non-boss levels.
- In this level, you'll fight five bosses in a row. However, every time you switch to another section of this level, you'll recover all of your health, so don't worry about not picking up the Heart items in the previous section.