How can I better balance my armies?

I'm a pretty new player, and I don't understand what I'm doing wrong with my armies.

I'm getting better at build orders, and I'm decent at making a strong economy. I have no problems really getting to the castle age, but when I produce my armies, I seem to fail.

I read before that its best to raid with smaller groups of units instead of waiting to build up a big army. That was my downfall before, but now it seems that my groups get destroyed by the AI.

I mostly use the swordsmen, and I mix in a group of skirmishers to try and fight off the enemy archers, but I'm struggling with balancing my armies to make them both survivable and producing them sustainably during the castle age.

Any advice to help me play better while playing more aggressively would be appreciated. Thanks.