One of the fundamental aspects of this game is establishing and maintaining a viable military force. Not only is the military necessary to defend your city against Looters, but also to conquer new Provinces, to take part in GvG Combat and Guild Expeditions (at Iron Age, or above), and to fulfill Advisor and Event Tasks.
My general philosophy is to build every unit available to me at my current level. I find that having a balanced force protecting my city gives me the best chance to defeat Looters when they come calling. The only exception to that mantra is the Bronze Age. Generally, you spend so little time in the Bronze Age, that constructing the camps for any Bronze Age Unit, with the exception of the Spearfighters (the Bronze Age Light Infantry Unit) is really a waste of time and resources. In fact, if possible, I would advise not even researching any other Bronze Age Unit until asked by a FoE Special Event to complete a Research, so at least you can be compensated for a useless Research. If you really want to have all the Bronze Age Units available to you, usually there is a "Special Deal" offered by FoE called the "Army Pack". For $3.99 USD, at the Bronze Age, you would get 2 Stone Throwers, 2 Slingers, 2 Warriors, and 2 Bronze Age Champions. Using that as your base force, along with any units you may be awarded doing Advisor or Event Tasks is more than enough to take care of all your Bronze Age military needs.
From time to time, you may win unattached units at a more advanced age than your current development, That is a special boon, as they can always be used in your City's Defense - and remember, units assigned to City Defense can never be killed by attacking armies - FoE automatically heals all units that are injured or killed in City Defense Combat for you automatically! For that reason, always have your most advanced units assigned to City Defense, and don't worry if they are Unattached.
So, now you are past the Bronze Age. You are seeking to build the Training Camps for all your new, advanced units. While you are doing that, do not tear down your Bronze Age Spearfighter's Camp - you will want to keep it. Why? One of the common Tasks during a FoE Event is to either Delete a Military Unit, Recruit a Military Unit, Tear Down a Military Camp, or Build a Military Camp. If there is no restriction on what age the unit or camp has to be (and often there are none), no other unit or military camp is cheaper to work with than the Bronze Age Spearfighter - so keep it around forever - just for that purpose.
Once you are able to construct the Progressive Era Alcatraz Great Building, you will find maintaining a strong military for Defense and Attack will become much easier. This Great Building produces, at Level 1, four random "Penal" Units for your Military per day, based upon which Military Camps are active in your City. Combined with the free Current Age units you recieve regularly in Guild Expeditions, you will probably soon be awash with Unassigned Military Units. This free influx of Units will make attacks on the Continental Map, in Guild Expeditions, and in your GvG battles far less costly, allowing you to concentrate your Resources on Research and Goods production.