While this is not my cup of tea (I dont particularly like facing hordes of enemies) and there is lots o new stuff, I like it because it's fun to play. The new stuff is easy to accommodate, there is an intelligent story, and it is fairly easy to make it an exploration game with combat rather than just fight after fight. The scenery is visually pleasing and the little touches like the exploding enemies delight. The revisiting levels on demand is an excellent feature. If you are low on O2 but want to explore, you can go forward until you find O2, fill up, an then go back and explore.
Marathon: Istoria is a single-player RPG scenario set in the Marathon universe and designed for the Aleph One engine. It pushes the engine to its limits by using advanced character progression, combat, and storytelling.
Customize your character by choosing from seven different player classes, each with their own unique active and passive abilities. Decide which conventional weapons you want to specialize in, and make use of a brand new arsenal of spells. Explore Istoria while discovering its secrets and how you fit in the world through a combination of terminals and communicating with the recently deceased. Istoria features an original soundtrack by Jon Irons.
NOTE: the "microphone" key in Aleph One has recently been changed to the "aux trigger" key, which is important to know for Istoria.