Need help in mid game

Hey all, I seem to struggle with mid game, right after hitting castle age. I usually plop down two tcs but then I kinda just randomly do shit I guess. All the while trying to produce vils. Right now I have been playing a lot of Celts, so I'll throw down some barracks and siege workshops.

Ideally though, what should I be doing? I always seemed to get outpaced in castle age.

Any good videos would be appreciated as well.

Thanks