The sole mission of this map is to stand through a period of time in which some units get evacuated from the town. This will happen automatically, scripted to the neutral player. So you don't have to be concerned about that, except that you have to take care of not blocking these units. Obviously, that would cost time, which you don't have.

On the front in the north-east, expect a series of heavy enemy attacks upon your front lines. Eventually, it will be impossible to hold these, but withdrawing is not an option either. You will need to hold them for as long as possible in order to build a second defense line in the middle of the map.

Your first and major concern should be the enemy artillery. Find a way to take them out as soon as you can. Doing this will definitely make it easier on your front troops to resist the attack for a much longer period of time. It is even possible to hold the front positions until the end of the evacuation, but then you'll have to fight real hard and keep replenishing your soldiers. Once the enemy artillery has been taken out, be sure to use your artillery on the waves of soldiers. Decimating them in larger numbers will certainly make your soldiers last longer.

The enemy/AI will win the game if it succeeds to get a unit into the town. You will win when you're able to sit out the whole evacuation process.