Need to build an army quick and than place it outside so the rival army can't destroy the forest and our facility. Moving at least 10 strong unit for this purpose and this is just a start, as long as the game advance always send some. Always watch this part of the map.
Move the archer so they can take advantage of the centralization of the force. So at all the same place on the wall. Possibly that is better to defend the south with the whole. Make some more for the north and the east. ASAP.
Always keep at least 10 soldier in center of the town to destroy the catapult after the rival soldier army have been taken down by the archers on the wall.
Grow, make money, sell goods for cash, buy food to feed the town. And make as much as solider as possible quickly.
So this is going to be my plan, the rest rally on luck and speed of the player.
Edit: Look like it is impossible to defend the forest from the first attack but maybe it's possible from the second.
No need to make cash until you want to give extra ration. Could be a good idea to do it but I've finish the mission without selling and buying any food. There is enough weat from the start to finish the mission.