Not really sure if this has been suggested before
Civic center trains conscripts, levies, milita, etc. (or simply basic rank units)
Low cost
Low combat stat (Low damage, low armor, low health, very inexperienced and easy to eliminate)
Quick to train (Really quick)
Basically just an early game unit and not really recommended for combat during mid or late game
Barracks/Archery range/Stables trains sufficiently trained soldiers (or advanced rank units) *Available during town phase
Moderate cost *additional metal cost
Moderate combat stat (Ideal in game combat units)
Moderate training time
These guys will be the ideal combat units in the game, able to hold of on their own if used wisely, their purpose is not to die alone but also put off a good fight.
Fortresses/Castles trains highly efficient combat units (or elite rank units) *Available during city phase
High cost *additional metal cost
High combat stat (Almost on par with champion units, but champion units still overpower them)
Above moderate training time (Only trains a little longer than those who trained at the barracks, both received good training, only that castlefolk could be upper class citizens that have drilled in the castle grounds)
These fighters are ideal for late game combat, they outmatch the regular fighters trained at the barracks and can hold off champions if used wisely.
One point here is to emphasize the role of basic rank units in building up economy, they are most vanilla of the citizen-soldiers and are tied really closely with the economy while still capable of providing military service under necessary circumstances, say if a territory is ambushed.
A player/commander can still train fresh citizens in the art of war, but this takes time, and one must be very clever in order to en masse a veteran army out of fresh recruits.
This makes the game interesting, well-off players are able to field decent soldiers while those who are falling in hard times must rely on the available militia for the defense of the territory, likewise a militia can still be effective if they are garrisoned or protected. This emphasizes that milita are quite good for local defense, and can also be trained as effective veterans if used cleverly during campaigns against the enemy.
There will be more variety of units for late game, this can help in developing more tactics and strategies against the enemy.
Plus there will be a good balance between the three ranks in the early to late phase of the game
A substantial amount of basic units will be deployed for resource gathering, it will be a waste of resource to invest on them as regular combat units
Majority of the population tasked with regular combat will then be invested in advanced rank units
It will be ideal to send tougher units during late game and this is where the elite ranks fits.
It will really depend in your strategy, you can carefully en masse elite units via promotion, as I've said, but this takes a lot of patience and time, you will have to use your healers more.