Apparently, Ladies Like Rail Shooters
Once you've made it through the underground highway tunnel at the end of Outskirts, you might think you're through with long, linear environments. Not so.
As you emerge from the tunnel opening into the daylight, the Sarge greets you with a present: a Scorpion tank. "Oh," he grins, "I know what the ladies like." as he gets behind the chain gun of a Pelican, a weapon so neat that we never get to use it ourselves.
Halo 2 never quite hits the level of a "rail shooter" where you're nothing but a moving crosshairs in a world where you have no control over your direction, but sometimes it seems close. True, you don't have to take the Scorpion: you can instead grab a Warthog and zoom right through the next set almost without shooting anything (if you don't count your Marine buddies. It's especially useful keeping a passenger with a rocket launcher here, especially when the Banshees attack from behind and then from the front after you make it halfway across the bridge.
As with every time you touch the Scorpion in Halo 2, you'll be hit with waves after waves of Ghosts, usually in pairs or larger groups, but even on Legendary you can make short work of them. The Banshees and an odd Phantom or two can cause a problem, but once you know when to expect them the cannon can take them out before they can even get into firing range. There is a wraith on the crown of the bridge, but just target the source of the plasma mortars and you can probably kill that before you can even see it properly.
This part of the level seems really large, but all too soon you relalize that most of what you're looking at is inaccessible to you; the bridge is surrounded by invisible walls that stop you from using a Banshee, should you manage to jack one, from doing much exploration away from the playable area, which is just over the surface of the bridge.
Once you reach the opposite side, I was thinking that perhaps there'd be a large outdoor area I could use the tank in-- something that gave me a choice of the direction and manner of approach to a battle, or perhaps even just something slightly more interesting than what we get... which is another section of tunnel.
It seems there are problems making the tank both powerful and interesting. Even on Legendary, once you've got the thing there's little that can stand up to you; and unless there's something particularly interesting to do, things can become a rather monotonous sequence of blowing up Ghosts, Banshees and the occasional Wraith.
The good news is that ground troops are more of a threat than in Halo 1: if an Elite gets close enough, he can board you, basically forcing you to either abandon the vehicle and fight on foot or get killed by a planted grenade: a very nice touch.
After a few more tunnel encounters sans scorpion you emerge into the larger of Halo 2's "sniper circle" areas, where I am encouraged rather strongly to wonder how much playtesting Legendary difficulty got.
In Heroic, the way this plays out is you creep out of the access tunnel with a few marines, snipe a few enemies, jump down, man the gauss hog's gun, drive around a bit shooting crap, and then move on to face a few tanks; if you're lucky, the hog and the marines survive all of that.
This is the way it plays out on Legendary:
You step out of the access way, melee the Jackal guarding the entrance, take his beam rifle, and get sniped by his buddy across the way. And if not, some of your marine buddies do get sniped.
You can run for cover and methodically take out all the snipers if you like, but there's a deadend ramp on the opposite side of the park that spawns about a dozen of them, one after the other, to replace one you've killed. By the time you've cleared them out, the Warthog and all the Marines are toast.
If you don't stop to take out the snipers, either you'll get sniped out of the 'hog yourself, or your driver will get sniped, leaving you a sitting duck.
The only smart way to play out the scenario is to take advantage of what appears to be a rather obvious bug in the way the encounter is scripted: when the hog stop for you to get in, kick out the driver and immediately head for the far side that leads to the Wraith encounter. You'll trigger a couple of Ghosts, but if you retreat back to the circular area (it looks like a hatch shell for an outdoor orchestra) all the snipers and other Covenant units (as well as any stranded marines) will be gone.
This strategy makes this part of the level much less fun, but having the gauss hog survive until the next set of encounters with the Wraiths outside the building the marines are holed up in makes that one a lot more fun-- the one with the dramatic music from the E3 realtime demo.
That encounter is a lot of fun and occurs in a fairly open area; but still there isn't much to alter, strategically speaking; you just drive around in circles hoping marines get in enough shots to eventually take out the Wraiths. Make sure you drive the hog yourself on Legendary or you're dead; also be sure you drive over the Jackal snipers on the overpasses or you'll run out of gunners fairly quickly and you'll have to revert to taking out the Wraiths from on foot or in a convenient ghost.
The last encounter on the level is to board the Scarab. Bungie tried to create some tension with the long walkway connecting several bridges that gives you a few chances at boarding it. However, after you've played it once you'll realize that the Scarab can't ever actually escape, so there's no rush here at all. In fact, unlike the first game, which ended with the Maw speedrun, there doesn't seem to be anything in Halo 2 that has to be done in any specific time frame. Cortana and other characters, of course, will prod you to move on at certain points, but usually when they do it's just annoying, such as when you're searching debris for good weapons or trying to do a trick.
Covenant units will pour out of the Scarab onto its exposed outer deck, where they are easily killed or simply blown off by the rocket launcher. Once you jump down on top, grenades can be used to take out any that remain inside the cockpit.
You will probably end up triggering this level's ending cutscene without even knowing it; unlike other areas, there's no button to press or anything explicit to do; as long as you step far enough inside the control room after all the Covenant are dead, you'll trigger it-- even if you've already run back out and on top of it, which can look confusing once the scene of the Scarab being destroyed plays. I imagine this was supposed to play out like the scene from Empire where Luke kills an AT-AT Walker by tossing an explosive into its gut, but since I hadn't done anything to precipitate it the experience was less dramatic than it might have been.
Metropolis is an enjoyable level, a good contrast to the old city atmosphere of Outskirts, plenty of vehicles to play with, and areas large enough to generate some replay value. However, for many, the level still seems a bit too linear and restricting, and the sequence of events on Earth comes to a premature end.