The key to protect your frontline headquarter is to upgrade an 800 exp skill called "rested and ready" in your EXP Upgrade panel. It allows you to instantly give the "sustain" order to one of your units and put it in front of your frontline headquarter to prevent the incoming attack from enemies cannon. Make sure you put a cannon behind the HQ and Monks in front.

After that you can either keep full presure attacking waves to quickly destory your enemies or create one more trading post, some resources building and cannons to steadily push forward.