Endless AI Spam - Unstoppable?

Hey all, I'm a pretty noob player, and I'm about 2 hours into my first ever game against the hard AI on a ludicrously sized map (I'm Britons and they are Magyars). We're right in the post imperial age - at this point I have around 20000-40000 of every resource and even producing units from dozens of different buildings doesn't bring down my count. However, I just can't beat the AI - I start by wasting their mostly cavalry army with a mix of fully upgraded halberdiers and longbow men, and once I'm in view of the castles I start producing trebs to knock them down. But every single time, the AI manages to somehow push me back with a massive wave of Magyar hussars and mangonels, and so the cycle repeats. I've tried everything I can think of - building secret blocks of buildings around their base to spam units, building up a massive army and attacking all at once, just straight up spamming from 20 or more buildings into their base - but nothing seems to work. Before I sink another 4 hours into this endless back and fro, does anyone have advice on what else I can do?