Among ground units, each foot unit is represented by a group of three militia; they all move together, in full animation, so that even with just one unit you can identify the front and back flanks – appropriate considerations from both the offensive and defensive perspective. As such, you are awarded a bonus when you fire into an enemy unit's flank. Adds Reynolds, it “also makes it look more realistic [in terms of the] number ratio of foot to horse soldiers” and without this you typically end up building as many horses as you would men which, disregarding the common sidetrack that men are not 'built', is simply not historically accurate.
As for the types of units themselves, given that your civilization does not find itself relegated to another people's history books, you will have at your disposal archers, spearmen, phalanx, various kinds of light infantry armed with javelins and both light and heavy cavalry such as the Byzantine/early Roman-era cataphract through Medieval Kings. For those unfamiliar with the cataphract, as was I until I did some surface-scratch research, it was truly the ultimate cavalry unit. They carried defensive armor that covered the whole body and often for the horse [ Reference: http://patriot.net/~townsend/GBoH/gboh-cataphract.html ]. Also present are early siege units, catapults, trebuchets, and early-model cannons. Each of these units upgrades as you move forward in time so you don't end up with antiquated units (and no need for a Leonardo's Workshop wonder equivalent, a là CivII).
“I never liked the situation of being in modern times and still hav[ing] spearman wondering around that you never quite got around to upgrading or simply hav[ing] no upgrades [available] so they are obsolete [yet] you still have them”, Reynolds chimes in on the subject. “[Still], you don't want to disband them because you spent good money on them but then you have to kind of run them off and crash them into somebody else's tank [or what have you], which never really made sense. We wanted to have the units [automatically] upgrade as you go forward [in RoN]”.
Most of the process in question is controlled by Age advances where to advance to the next age, your civilization must have reached a certain knowledge level in different technological categories where only then can you perform specialized research that leads you into a new Age. All of these upgrades are performed through the buildings in your cities that allowed you to carry out the unit's construction in the first place, e.g. Barracks, Stable, Siege Factory, Dock, Air Base etc. The next couple of sections are dedicated to following along with a few examples Reynolds demonstrated to me to give both an idea of the scope of the varying types of units available as well as how the “auto-upgrade” desire is incorporated into Rise of Nations.
BY LAND, BY SEA, BY AIR: MILITARY UPGRADES
For example, let's say you start with a hoplite unit, a basic, early heavy infantry unit staple of the Classic Age. From here, you can move on up to a Greek-style Phalanx and – assuming it's not a unique unit – you can further update to a Pikemen, then Elite Pikemen and as you move into the Modern era your said unit may be allowed to acquire guns and transform itself into the likes of a Phusalear. The last unit stated carried a special type of gun which allowed you guard the artillery. From here, in a continuing line of units well matched against horses, you could get your hands on an anti-tank rifle and eventually move forward to the present-day bazooka.
Rise Up: Reynolds On RoN 