To have the buildings lose HP from the start, make a trigger with the starting state on, looping off, and objective off. In the trigger, use the effect Damage Object, and under set objects, click on the building. Type the amount of damage you want to give the building. The larger the percent of HP lost from the building, the greater the flames will be.
As for which civilizations to use, I like going with any Northern European nation, ie Teutons, Goths, Vikings... because their feudal age buildings are really grey and worn looking, which is perfect for a winter scene.
Your battle is looking a little lackluster. For the battle I'd make the enemy stronger than you, since you're defending. Also from the screen there was only one trebuchet. Consider making more trebuchets, and put them behind some kind of palisade wall to give them some protection, and it looks very cool too.
The corners of the palisades with the flags may look a little ugly, so simply remove them with triggers. Use the effect Remove Object and click the flagged corners under set object to remove them.
Do you want this to be a long, drawn out battle, or is it simply introductory? If you want it to be long, consider spawning the enemy units, and giving your men a little more HP.
Hope I can help you out.
I strayed out of thought and time. Stars wheeled overhead and everyday was as long as a life-age of the earth. But it was not the end. I've been sent back until my task is done...
... I come back to you now, at the turn of the tide.