Rise of Nations is the closest RTS to a historic based empire building 4x game.
So what are the essential Empire Building elements does Rise of Nations does not have.
1. Running a large empire can be quite a burden. The larger the empire gets, the more it's needs are to maintain itself. One of the biggest burdens of running an empire is running a large armed force that needs to be armed, and fed. In fact some the cost of running a large army can be so great that it can bankrupt a nation that doesn't run it's finances well. The way that Empire Building games realistically represent this is to create a burden on the player's economy called upkeep. The larger the armed force, the greater the burden. This forces the player to plan the growth of their economy as well as the expansion of the armed forces so that the burden of upkeep doesn't bring the empire progression to a halt. Rise of Nations has no upkeep or other kind of burden. To limit the size of a player's armed forces it uses a unit cap, an common element in RTS games that can easily be overcomed through research or building of certain structures.
2. An important aspect of running an empire is managing your people, which entails make sure that the population is growing and is consistently happy. This keeps the cities productive, keeps the economy growing, and prevents civil problems. Rise of Nations has none of that, so you are truthfully not running an empire.
So what features does Rise of Nation have that make it so close to an empire builder:
1. A Research Tech Tree
Once you build a library near one of your cities, you can start climbing the research tree, with each technology having requirements of previous research on the tree. This an essential part of an empire building game.
2. Multiple Diplomatic Options: An empire does not always have to go to war to continue to develop.
3. Expensive Wonders that you can build that provides your nation with specific benefits.
4. The ability to expand by building new cities.
etc.
Another words, there are more small elements of the empire building games that it has then the lack of the big elements that I mention.
All in all, this game is the closest historical based RTS game to empire building games, which makes it one of the best RTS games ever to be published.