Campaign is kicking my ass. Advice/guides?

I just had to cheat to beat Joan's 3rd mission, which is supposed to be an easy mission.

After a couple restarts and quick research, I found that the best place to camp was southeast corner. There are at least 3 enemy armies and they are always attacking, so I built walls at my chokepoints, though as I cut trees, I had to build more walls. Unfortunately my knights had a habbit of opening the god damn gates which made me need to micro.

I slowly sent my blob and about 2 to 3 rams to attack the red castle just to the northeast. I built a castle right outside of their gate to heal most of my wounded. Then as soon as I killed their castle, the other AI just started sending waves and waves of enemies and I was running low on resources by then. I tried to repeat the strat on orange, but purple snuck some rams through the hole my lumberjacks made in the trees and destroyed my TC. I was all out of stone, and I just snapped and cheated.

This all took so much time to do and it was frustrating as I wasn't sure what I could be doing better. The only thing I thought of was to just not even build walls and just mass more units and build even MORE rams... whatever, on to the next one.

Any good guides/ tips y'all know of to help with campaign especially? I don't know what units to focus on so I end up building an even mix. My xbows tend to die fast, and it is really hard to micro around balista.