Space Crusader is a level and location cut from Serious Sam: The First Encounter.
Space Crusader is a level that was supposed to be between the Rock Planet and Sky City locations. It's likely it would serve as an intermission of sorts that would help the player adjust to the gameplay in Sky City.
No definite pictures of Space Crusader exist. There are several pre-release images showing space-related locations, such as space stations and a planet with a space-like background, but there's no way to determine which level might've been Space Crusader.
Space Crusader was completely removed from the final version. The only sign of it's existence is it being in a level list found in Test 1's files.