Marathon 4: The C-Files Redux v2

The C-Files is a bunch of levels strung together, made by a couple of kids experimenting with Forge for the first time. The levels aren't that great, but it can be used to test your Marathon endurance, since there are no save terms, or terminals at all.

The Redux version puts the levels in their originally-intended linear path instead of the first release's chaos, plus adds an introduction level to the beginning.

The second installment of the series, Cyle Returns, is far better. It actually includes a story line, terminals, decent maps, etc.