This is certainly one of the most beautiful Marathon levels ever created, and one of the most unusual. There are no weapons, no health, no monsters, no danger (beyond spending too much time underwater), and nearly no switches or other interactive elements. There's just a vast and varied landscape to explore and jump around in, to find more cool things to look at and eventually a terminal or two, a vague objective, and an exit point. There are twisting caverns, a lighthouse on a beach, a huge verdant canyon to jump around in, several groves of trees and one very big one, a hidden temple of some sort, an underground water-powered mechanism, and more. However, the fact that there's virtually nothing to slow you down (besides the occasional trip back to the beginning of the level) means that it's pretty quick to play.
Is it the best level ever made? No, because to me fighting monsters and fulfilling some sort of objective are essential parts of Marathon, and the best map ever would have to properly integrate those aspects as well. An exploration-only map is often used as an interlude between combat levels in other scenarios, but they are made deliberately short because, as we saw many times in the original TI, looking at nice map design is not enough to carry the game for long. (One may point to Missed Island as a successful monsterless map, but it has a tremendous number of interactive elements and was made from entirely different principles in general than SatG.)
One more thing to keep in mind when experiencing this level (at least for me) is that when one first encounters it, it's not immediately obvious that there is virtually no danger. This lends the quiet greenery and dark caves a rather more foreboding air than the author may have intended, as it's hard to let go of the expectation that the monsters are waiting around that next corner. Of course, I may be wrong about "more than the author intended", as some areas of the level (such as the final catacombs beneath the tree, and the terminals) seem designed to create a feeling of unease.
The terminals on this level are quite different from those in TI. The "main" terminal, in the lighthouse, talks about Leonardo's (I assume) observations of S'bhuth's weaknesses and the introduction of the mysterious last manuscript. It's rather nitpicky, but I wish the terminal had replaced "find the tree" with "return to the tree" or not mentioned the tree at all, as it's not possible to reach the terminal without seeing the Tree on the way. The secret terminal contains a poem by Yeats named The Second Coming, although I doubt the last remark is by Leonardo as Yeats lived several hundred years later.
There's one minor bug with this level: The slipgate is always active and will always teleport you out of the level regardless of whether you've turned the light on, because although the light switch is a repair switch, the level itself has no objective set.
