There is a couple of options to beat Arambai tho,If you are wondering how to beat arambai in the castle age, you do not have any options whatsoever. The unit is very imbalanced and the devs like it this way.
There is a couple of options to beat Arambai tho,
1. Prefent the player from getting to it.
2. Play full skirms and mangonels.
3. Depending on Map and if its 1v1 or TG a single castle can block of the entire arambai push.
4. A couple of UU beat arambai in cst age aswell (Rattan archers,Camel Archers,Mangudai) ofc all if microed properly and if u have enough of them.
If u habe 20 rattans and a mangonel or 2 u destroy arambai with proper micro cus the arambai player loses 6 arambai for killing 1 mangonelRattans get wrecked by arambai in castle age, and mangudai u rely on a lot of upgrades, so only in late castle age they do ok.
Skirms with ballistic works fine and mangonels can work mixed with some other army as well.
Arambai are a bit strong in castle age but the civ still not considered ''great'' and theres a good reason for that 11. They're weak in post imp, they dont have amazing eco and not a lot of options or easy transitions from castle age army to imp army. And only +1 defence on their ranged units.
They should have an adventage at some point, imo.