Regret is a level that I like almost in spite of myself. It pulls off an impressive sleight of hand, managing to create a fair impression of size and continuity in a mission that contains relatively little playable surface (comparatively) with areas that are linked only by pieces of moving geometry that the player has only rudimentary control over.
At this point, it goes without saying that the atmospheric details of the settings-- the overgrown temple built over by Forerunner structures, making the player wonder who actually built the Halo installations and why-- are exquisitely detailed. And this time, unlike many other such environments, you'll probably take some time to actually look at it.
As you approach the first structure, with a ramp leading down to a door, you'll probably have a beam rifle. If you don't, walk back and get one; at the end of the final tough spot from Delta Halo there's a rack with two of them in it.
My first impression looking at that door, unfortunately, is that it is going lock behind you. That's true, it does-- although not immediately. However, in any case, the entire structure tended to make me wary, and the designers obviously wanted to make you snipe away at all the enemies you see (and in the process thoroughly inspect the geometry they spent so much time texturing and aging) before entering the temple.
Once you do commit to going inside, the level becomes a sequence of similar structures-- old temples inside, modern structures with open-air surfaces and ramps outside, two sets of gondolas that travel over the water to connect two such stuctures (there are four total in the level, including the initial one where you see a hologram of Regret, and the final one at the center of the lake, where you finally encounter Regret himself) and two underwater elevators that link the second and third of the temple structures with a large underwater facility that has a room-sized hologram of Regret.
The environments are novel enough to distract the player from the fact that each of these journeys is one way. The elevators and gondolas can be activated by touching a panel, like most of the doorways and elevators in the first game. However, unlike most of those elevators and doorways, none of these conveyances will let you backtrack. There's no reason to backtrack, of course, and perhaps this is just a peculiar habit of mine and those who like to do tricks; but the fact that I can, at least, traverse the entire playable area of a level in any direction I like at any time. It makes the level feel like a real space.
There are some parts that do create that impression. The gondola sequences are great; they create novel ways of encountering enemies, as the gondolas usually arrive with Covenant on board, and halfway through their travel to the opposite structure, they pass a gondala traveling the other way briefly-- providing another opportunity for a combat encounter. They do a good job at faking continuity, though. The height of the gondola creates a fairly believable reason for being restricted to just this small strip of playable area between structures; and once you get off at the end of its travel, you can zoom in with a scoped weapon and see the other gondola resting on the far side. However, the fact that you can't push the button again and return breaks the verisimilitude; it emphasizes the fact that this is a game environment created by a designer for the purposes of moving the player from encounter A to encounter B-- rather than a real, living, breathing world that you, the player, happens to have stumbled into.
Beyond that there is no apparent line of sight between the two gondola areas and the underwater structure. Once you emerge from it you find yourself in a similar-looking area; a lake with a gondola going out to a temple. It looks very much like the area you left before going into the underwater structure. But looking around it's difficult to be sure where you are in relation to where you were. The fact that the temple is open to the air makes one immediately wonder if it might not have been better just to fly there in a Pelican, or, for instance, hijack a Banshee. Of course, in this level there aren't any banshees to hijack until the very end, and even then I'm not sure if players have managed it yet. Again, the impression of continuity created in the mind of the player is shattered the moment he or she pauses to wonder if there is another way to arrive at the intended destination only to find that of the many possible ways (walking, swimming, flying) only one is open, the one the designers intended you to take, and it's a one-way ride.
However, the gondola encounters themselves are quite fun. The music that plays is clearly the new version of "On A Pale Horse" and it lends a lot of drama and tension to the sequence; not that on Legendary it needs any more than it already has, as you'll be attacked by banshees, shot at by Grunts with fuelrod guns, and flanked by flying Ranger Elites.
Finally you'll arrive at your ultimate destination, the temple where Regret is delivering his apparently superfluous sermon, and the first real "boss battle" of the game. Unlike the Scarab, that you only had to board and clear out to defeat, Regret is an enemy that must be attacked in a specific way, and cannot be defeated by normal means. I won't spoil it here, even though I assume most players already know it by now. Sufficed to say that while it is cool-looking and makes use of a new "feature" of Halo 2's gameplay mechanics, it breaks verisimilitude to a frightening degree. And the "unique attacks" promised for the individual prophets turns out to be twin beams similar to the new ones given to the Hunters... only a different color.
Delta Halo and Regret form two sides of a one-way ticket through the gloriously detailed lake and forest regions of Installation 05, with small detours underwater but no swimming or flying allowed. However, make sure you soak it up, because Master Chief won't see much sun for the rest of the game.