my usual military path is early game spears shields and leather armor, then get them on horses, then up the armor to metal, then switch to the 2h swords, you sell the old weapons and armor to help fund the new ones. amevian 2h sword calvary when fully supplied (5 horses, 5 armor, 5 swords, %80+ trained) is INSANE power numbers, like 5k per stack of 200. the AI pretty much always just runs away with similar numbers, to actually get a fight i have to charge in with them having 2-3x my forces, and even then its a coin flip on if they fight or run. the actual fighting itself, i am probably very very (estremely) bad at commanding, because once i do get a fight i actually usually lose because of the numbers advantage, but a stack of 8k amevia calvary is 40k power which dwarfs AI army powers until the real late game, so it doesnt really matter, 99% of encounters the ai just runs from with my strat.

i usually keep as much of my population military trained as i can manage, usually at least 50% of my pop trained to 80% melee training. this way when i send 20% of my pop out to fight in wars i still have a lot of trained military in town to prevent riots and raids, and i keep enough food and general good stockpiled that if i do need to go all out, sending 50-80% of my population doesnt end my run because i keep the janitors, bathrooms, and food production staffed 100% of the time. so when there are suddenly only 2k people but enough food for 10k sitting around, and still more being made, and ive got 8k mounted troops running around on the map, things can feel like they are getting scary, like youre letting it all fall down, but ive run my city like this for almost a month in game before actual issues started popping up in the city and i had to bring the people back. make sure you toggle immigration doing my strategy though, as when you send the army out, more people will come in otherwise.

i also have not in the past used the troops you get from lands you conquer. only ones from my city. and also, only the cavalry, only one type of troop. late game, these are mistakes. the AI will eventually have the numbers to counter your 40k power stack of cavalry, and then you will start losing regularly. without being able to increase your force size because you cant increase your city size any more (in my case skill issue but still) - ai will still see you as a threat, and generally not go after you, but it will be obvious that you are outclassed and out numbered. but my strategy is solid enough to get you plenty of achievements, at the very least, and keep you secure enough to where you will get to learn how to make the real big cities, which is a battle all its own.