Setup/Level Warp

The Level Warp window is much like the multiplayer counterpart for starting games.

The first options allow you to select the specific game, for example between Doom and Doom II. In Hexen, you will be able to also select a character class to play as. The skill option changes the game's difficulty, and the level warp will allow you to start at any level of your choosing.

Monster options
The monster options are also identical to the multiplayer counterpart:

Monsters enabled will toggle whether monsters even spawn in the level. If you just want to look at a level's architecture, this is handy.

Fast monsters will cause the monsters to move and attack at a much faster rate than normal.

Respawning monsters will cause the monsters to resurrect 30 seconds after you kill them. This can make levels much more difficulty, and potentially near-impossible to beat as you run out of ammo.

Advanced
You can also add extra command line parameters and WAD files; see Setup/Extra dialogs.