Border Tactics
Assault
Going north means you will be on a large open stretch. There are a few good footholds that, if you can secure, will help you in your advance. Some of these would be the North Hill Ruins (the ruins by the elm tree) and the square delapitated building to the north of the elm tree. A rush is the most effective tactic. Taking the North Defence Hill ruins will be a challenge. An alternative north route to take is to go north, then swing east through the north hill ruins and down to the destroyed building.
Going South gives you the advantage of elevation and cover, however south is hard to attack because of the urban setting. Snipers can be hidden in any of the buildings in the valley. A south assault can be more direct, but most be done slowly to be completed successfully.
A central assault is a more dynamic situation. Often you will be pinned down behind a square white building in the central valley if you try to go directly central. While in the central valley, you can swing left and go north up the hill to Town Hall. From here, you can take out opposition that might hinder your advance. Also, running from the hill to the blue building provides cover from the "Exploit" building.
Defense
The best place to cover the Southern passes is to enter the building with the two windows and crouch. If you stay stationary in the crouched position, especially on the right side, your head is almost invisible to their rushers. Scan the hills and the South pass and take out any assaulters, and watch out for a flank later in the round.
The next best place is the balcony to the left. Not as much cover as the first position, but definately worth sending an extra man.
Covering the North you have many options, but all get you picked off rather quickly. There are the walls/destroyed buildings to provide cover. Keep an eye on the hills, the Town Hall, and the destroyed building.