There's about 4 Combine and 3 hoppers spread throughout the courtyard. You'll also be fighting enemies at different heights, so you'll have to stay alert. They also love throwing grenades at you, so be ready to punt them away with the gravity gun. Expect to take heavy losses to your squad as you move forward to the one open door. It's the one that's right below the raised draw bridge.

click to enlarge
Go upstairs and kill a few headcrabs until you get up to the drawbridge itself. Kill the Combine guarding the switch if you didn't already. Take cover by the bridge and use the magnum or the AR2 to clear the rooftops to the right and left. Then focus on clearing the building in front of you. Lower the bridge and start to cross. One more should run out, so kill him. Then go inside and kill the two Combine guarding the entrance to their bombed out base.
Round the corner and kill the two soldiers just ahead. Then kill the one behind the pillar to the left. There's a section with two soldiers ahead, but it's blocked off by fallen concrete. Shoot them from the cover of the holes, and walk past the section to get to the stairs. Shoot the barnacle guarding the first set, kill the barnacle guarding the second set, kill a zombie that might appear, and then drop through the hole in the floor to the sealed off area.
Go forward and kill the flaming fast zombie that runs toward you. Just hang back for a second once you see the Combine ahead. They are fighting a number of zombies and fast zombies. They also like to use grenades at an idiotic range, so the one up top will probably blow up his buddies below.
Go through the hole, but be very careful about any surviving Combine. Go down the little set of stairs and up the next one. Go to the only open room and look down to see some turrets killing zombies. Walk around and throw a grenade through the hole to knock over the turrets. You might want to use the alternative fire to exercise a bit more control. Drop down and use the gravity gun if it doesn't work perfectly. There are two chargers on the wall, so charge up your health and your suit.