Bruce "Hippieman" Morrison at Freeverse, the developer that brought Bungie's Marathon: Durandal to Xbox Live Arcade, has been working on a walkthrough of the game. While of course there is always the Marathon Spoiler Guide, which covers all three games, Marathon, Marathon 2: Durandal and Marathon: Infinity, Hippieman's walkthrough includes screenshots of environments, whereas the Spoiler Guide features only maps.

Currently Hippieman's walkthrough is a work in progress; you can download the latest version to see how far he's gotten; he announced the file in this thread in the Xbox forums.

For those who need a different kind of help with the game-- the kind needed to wring all 200 gamerscore points out of it-- Xyjar posted a Marathon: Durandal Achievement Guide.