Half-Life 2 Walkthrough -> Follow Freeman!
Let me remind you that to leave your squad, you need to press "C" (by default). You don't need to turn to face your squad (!), but if this does happen and the "team moving" message appears, press "C" again to make the message disappear. To call your squad back, press "C" again (the corresponding message should appear).
Marathon
Run down the street (holding down Shift) in a zigzag pattern, from corner to corner, dodging bullets. Run right up to the sniper's window and toss him a grenade. Then return to the turnoff to another street, at the end of which there is also a sniper hiding. We don't lead the squad, leaving them where they are for now.
Approaching the rubble, we aim for the crossbar of the upper window, or slightly lower (the second one is above the sniper), swing, and after about three seconds, throw the grenade. Then we return the squad. Incidentally, there's a girl standing on the corner behind the stained-glass windows—a potential member of our squad.
After that, zombies will start approaching from all sides out of the water. After firing, we go up the stairs to the right of the dying rebel to the very top. We leave the squad on the stairs and, after waiting for the snipers to shoot all the zombies, throw grenades at them. We go up one floor and throw grenades at two more snipers.
Suppression
We go inside. There are three barrels in the courtyard; use them against the turret and the soldiers on the porch to the left. We go up. Barney says there's some kind of museum across the street, but a suppressor device has been installed there, and while it's active, there's no point in attacking the Combine any further.
So we need to sneak into this building and disable the suppressor. But behind the door Barney opens is a security camera, and it's impossible to get past it unnoticed. Even if we disable it, the alarm will still go off. So, let's head down.
Throw the first mine at the turret on the left, the second anywhere. Exit. Reach the other side of the street as quickly as possible. If you do it slowly, there's a good chance we'll be the only ones left in the squad. And without Barney, you can't even play. Go around the blue container to the right, and then immediately (!) to the left.
We walk past the porch of the former museum toward the force field. The force field is deactivated the same way—by pulling the plug with the gravity gun or by causing an explosion. We enter. Behind the green laser beams, similar to sniper beams, is a ground turret.
Generators Again
We're heading across the bridge to the left of the room with the first generator. Straight ahead is a ground turret. To the right, at the end of the hall, there are a lot of Alliance soldiers and elites, but it's easier to kill them now, when they're all grouped together, than later, when they start attacking from different directions.
Before reaching the hall, there's a booby-trapped room to the left. Plus, it has a powerful security system, and if even one laser beam intersects, several powerful turrets will emerge from the ceiling.
So, we cross the room without crossing the red beams. You can safely detonate the mines (but don't touch the photographs—if you remove them from the wall and they cross the beams, the security system will activate). After reaching the opposite exit via the pipes and cabinets, we disable the security system and, if necessary, return for the squad and Barney.
They will shoot at us through the holes in the walls for a while, then they will start welding one of the doors. Alliance soldiers will soon knock it down. There are several mines in the room; you can place them in front of the doors. You can simply throw them into the crowd of soldiers immediately after the door is knocked down.
After shooting everyone, we leave the squad in the room and head left toward the ground turrets. By destroying the third generator, we'll firstly remove all the force fields and secondly, disable the suppressor.
We turn right and climb up to the roof. There's a chest with grenade launcher shells on the roof. We'll need it soon. We drop down to the second roof, after which a stormtrooper will appear. We shoot it down.
Next, head to the gate, to the right of which is an armor and health recharger. Soon, soldiers will disembark and open the gate. Head to the bridge and activate it with the button on the left. Enter the opposite building and go down.