Super Mario World/Level types

There are 7 main types of levels in Super Mario World. With a few exceptions, each level falls into one of these categories.

Above-ground
This is the most basic level type. It is characterized by a relatively small amount of ditch space and large amount of "terra firma." Because of this, the chances of falling into a ditch after falling off a platform are slim. There is no barrier at the top of the level, and often secret items can be found floating up high. An example is Yoshi's Island 2.

Elevated
Elevated levels are distinguished from above-ground levels by a large amount of ditch space compared to "terra firma." Hence it is more likely than not that falling off a platform will result in falling into a ditch and dying, and so it is most advantageous to have a cape in order to avoid ditches. The distinction between elevated and above-ground levels can become somewhat blurred, and the most obvious difference is the background music; however, some levels, such as Gnarly, would be classified as above-ground instead of elevated levels, despite having different background music from most above-ground levels.

Underground
Underground levels take place underground, with a clearly defined ceiling and distinctive background music. Some enemies, such as Spike Tops and Blarggs, are found only in underground levels. An example is Donut Plains 2.

Underwater
These levels take place underwater, and again have a distinguishing background music. Most water-dwelling enemies such as Blurps are found only in underwater levels. Note that underwater levels do not simply have water: in order for a level to be classified as an underwater level, it must be impossible to get out of the water. For example, Donut Secret 1 is an underwater level, whereas Yoshi's Island 4 is not.

Ghost House
Ghost houses are easy to identify: they are the only levels with ghosts ("Boo" Buddies, Fishin' Boos, Eeries, and Big "Boo"s). Ghost houses often have less obvious exits than most other levels, and frequently also have secret exits. The game can be saved after completing a ghost house.

Castle
Castles are the dwelling places of Bowser and the Koopa Kids, and are in general more difficult than typical levels. There are 8 castles in the game. Castles never have secret exits, and always require defeating a boss at the end. Like ghost houses, completing a castle enables the game to be saved. A castle which has already been completed can be replayed in 1-player mode by simultaneously pressing the R and L buttons while standing on the ruins of the castle (doing this in 2-player mode brings up the life transfer utility).

Fortress
Fortresses are exactly like castles except that instead of the bosses being Koopas, they are always Reznor.