The core experience in Age of Empires Online should be very familiar to fans of the genre, particularly those who played previous Age of Empires titles. Worker units gather all manner of resources, which are used to produce new units, make new buildings, or develop technologies. One of the distinguishing characteristics of Age of Empires is the variety of materials to obtain; instead of merely spice or tiberium, or even minerals and gas, you'll need to balance the acquisition of four different resource types: food, wood, gold, and stone.
As a game progresses, you can advance through different Ages, each one unlocking new units and buildings. Once you have taken care of your economy, you can focus on building up your army, producing all sorts of different units in your quest for domination. There's nothing really new as far as the core gameplay is concerned; some may see this as a problem, but there are many other improvements to the game,and besides, why fix something if it works?
The real innovation in Age of Empires Online is the integration of MMO-like components. Instead of the typical fare where you have a character that earns levels, your capital city, instead, is what grows slowly over time. The capital serves as a sort of hub, where you can take on quests, purchase upgrades, and all manner of other activities. The game starts off quite simply, with one quest at a time, gradually building you up to a point where you may have many quests available at any given time. Again, it's very much like MMOs in this regard. When you take on a quest, it launches the corresponding scenario. Most quests require you to build up from scratch in order to tackle a goal. You might have to simply destroy a certain amount of enemy buildings, take down a tough opposing unit, or simply defend yourself for a given amount of time. The quests are set up so that you gradually learn about all the different units, and the game's difficulty curve is very smooth as a result.
As you complete quests, you earn rewards, primarily experience points but also including raw materials used in crafting, gold to purchase items from stores, and more. When you level up, you earn points that can be spent unlocking items in the technology tree. The system works very much like the talent trees in, say, Diablo 2. There are three main areas to choose from: economy, military, and utility. You might spend points to allow your gatherers to carry more goods at once, or unlock an archery range in order to build bowmen. As you would expect, this levelling up system is very addicting. You'll find yourself playing "just one more game" in order to unlock that shiny new unit that will allow you to dominate in battle. The customization of your abilities extends to all manner of other areas, as well, including crafting, special items that can be used to your advantage in quests, and even cosmetic features like hedges and statues that you can use to increase your capital city's cool factor.