The Cairo Station level is where you really start to feel nostalgic, not only for Halo 1 but for much older Bungie games. The parallels to the first Halo game start to become apparent, as well as the influence of ideas and themes from the Marathon series.
Leading up to this point we've gotten a cutscene to establish the setting, and a short tutorial on the controls and the Mjolnir suit-- exactly what we got in Halo 1, except that the cutscene was split between UNSC and Covenant perspectives. After that came the tutorial on the shields and aiming, just like in the cryo bay on the Pillar of Autumn.
After that, we had to repel Covenant boarders on the PoA and then escape to the surface before the ship crashes. In Halo 2, we have to repel Covenant boarders on the Cairo and then escape before a Covenant bomb destroys the station. The similarities are quite obvious and striking; the bomb being the only real wrinkle, and it's a small one at that, given that once you reach it the level is over and the cutscene plays.
This scenario also allows the player to see right up front how different the physics of Halo 2 are from the previous game. Where on the PoA, the Covenant entered the ship through the escape pod airlocks, cleverly only requiring Bungie to model the inside of the boarding ships, this time the Covenant drill through the station's walls and observation windows. It's quite a nice touch to see the ships outside and the melted holes through which the boarding ship's proboscis protrudes.
However, it also produces one of the game's second poke-in-the-eye moments, visually. When you defeat one group of Covenant, another group pours out of the boarding ship. But the breaches are about halfway up the wall, meaning that Covenant units, especially Grunts, seem to bounce out of the opening like gumballs, rolling and bouncing in an almost comical manner; their limbs seem to be frozen in space until they hit the surface of the deck, which at distance makes them almost look like two dimensional sprites from some ancient shoot-em-up. It's not that the physics engine isn't good, or perhaps even better or at least more comprehensive than Halo 1's; but Halo 2 will counter and frustrate any experienced Halo player's expectations of how objects will behave at almost every turn; and the way units jump, fall, bounce and roll is one of the earliest examples of this. Covenant corpses crumple and fall in more detailed and realistic ways, even moving slightly after death, especially if the body is on an inclined surface; but the whole process has such a different feel that it gives a player pause.
Another issue that becomes apparent in the Cairo level is detail. Whereas Halo 1 had some detailed objects-- units, vehicles, weapons, and certain portions of the scenery, interspersed with some rather large and basic objects (snowscapes, grassy hills, etc) almost everything in Halo 2, especially in Cairo Station as wel as other levels, has an almost confusing level of detail. Detailed objects in Halo 1 were almost always signs that screamed out "pay attention to this item, I'm either important or interesting looking". But everything screams that in Halo 2; so many items have detailed textures and move and sway that it's hard to know what to pay attention to sometimes and hard to locate targets in among everything else; especially true when drones are flying against a detailed background. Of course, there's nothing wrong with this; in fact, one might even say it lends the game a greater degree of verisimilitude. But it is another thing that Halo 1 players will need to adjust to.
Cairo Station is basically a corridor crawl, similar to the interior areas of Truth and Reconciliation from the first game, with its mix of small, cramped spaces with limited options for cover and wide open areas. When the battle moves outside into vacuum for a short while, the look of the outside of the station, the large pieces of moving geometry, and the change in the music and sound, will all scream "Marathon" to old Bungie fans. And the effect is quite pleasing, I must say. The only problem really is-- and this is something that has been said and will be said about Halo 2 in a number of ways-- is that it ends just when it starts to get going. By the time you get outside the station, you're already more than halfway through the level, having cleared the hangars of boarders. The first exteriors with the flying Elites is quite intimidating the first time you encounter it-- until you realize that the distance to be covered is so short that on anything less than legendary you can get through it fairly easily by just waiting a bit for the Elites to wander away; plus, there are several places you can hide inside what look like cargo containers.
There's one more exterior area like that, near the Mac gun mechanism that is definitely Halo 2's first real "oh, wow" moment-- fierce combat, sound and motion everywhere, a real feeling of chaos and the thrill of mastering a confusing situation. The only problem is that it's almost over; the next major encounter takes you to the bomb room and the level ends.
As a counterpoint to the sometimes distracting visuals, it is well worth noting that the sound and music in Cairo Station are fantastic. There are two moments in particular that work extremely well. When you pass through the airlock that leads outside to the area where you first encounter the flying "Ranger" Elites, the sound becomes deadened-- a decent compromise between a realistic behavior (where sound doesn't travel at all in a vacuum) and giving the player appropriate audio feedback. In addition, there's a subtle and surreal piece of synthesizer music that plays in the background of that encounter; something quite different from the rest of the soundtrack that lends it just the right mood. The music changes to a bass-heavy bit of techno when you enter the fire control room where the Covenant have placed the bomb; it does a great job of building up tension.
The idea of the Covenant boarding an orbital defense station to disable a Mac gun makes perfect sense inside Halo 2's backstory; it follows a parallel structure with the previous game, and thus makes perfect sense. However, all too soon it begins to feel all too familiar and yet not familiar at the same time. Even when it's enjoyable, Halo 2 will have you wondering why so much has changed, and reflecting that the more things change, the more they stay the same.