This is very good real-time strategy game with big focus on tactics (unit types, facing, flanking, upcoming - currently beta tree - ambush system) and logistics (need to connect assets, such as cities, mines, farms, woodcutters with land or water connections, through which food and wood is transported, and depending on length/difficulty of connection, there is tax - longer routes cost more in gold, but also have % of wood/food/gold production lost).
Another part of logistics is recruits - each city has capped amount of recruits pool, which regenerates over time. Your units need to be in supply range of fort or city with land connection to their home city to be able to draw recruits to replace casualties (further from home city, slower the reinfoircements. You can change unit's home city, but (probably) only within other cities of the same tribe.
Big part of the game is unit experience, which is what will keep you playing mostly.
You are member of particular civilization and tribe. Your native cities are loyal, but any conquered cities need either garrison buildings, garrison, and/or hostages put somewhere else to prevent it from rebelling (don't worry it's not too complex and you don't need to pay constant attention to it). Conquering another nation can mean getting their wood, gold and food sources, their recruitment pool, but depending on their bonuses you get access to units with combat bonus of their native faction or terrain capability of that faction, or even to different unit types (if you conquer roman city as barbarian, you get access to roman legionaires, which have very different stats).
There are some differences between nations/tribes, some have larger number of native cities, others have some bonuses (meelee/ranged combat bonus, terrain-specific unit capability on recruitment).
Amount of unit types is not huge - skirmishers, spearmen, slingers, cavalry, javelinmen, archers for barbarians, additional infantry and cavalry of roman factions. After earning experience in battle (or small amount on recruitment with some building upgrades build) you can (and should) spend this on purchasing officers for that unit, up to four (non-detachable) officers of four levels plus (detachable) general. These can provide significant bonuses to offensive and defensive capabilities of unit, morale, logistics, terrain-passage, siege, scouting, and probably more bonuses(typically you'll want combat bonuses first for meelee units, ranged bonus for ranged units). Generals work same as officers, but can be detached and moved to another unit or to govern some city (not sure about effects of that, maybe is still work in progress).
There is lot of content in the game already, and game is very good, although there are still things to be done (AI is good in handling some tasks but not so good with others). Good news is that developers are working actively on the game, and judging from support history from original Hegemony game (which was focused on Greece) they can be trusted to do so for years and years to come.
Compared to original Hegemony, there is less variation in units, because it takes place largely in homogenic area of Gaul.