Hey, just thought I'd mention a RTS I play a lot in my spare time.
It's called Call of Roma. Free to play. The theme is Ancient Roman Empire (Republic period).
General concept of the game is to build up your city, and technologies which increase production of resources and improve strength of city wall ect. There are several troop units, ranging in power, cost and building level to make, all with their own benefits. You can attack other players, plunder their resources or colonise their cities to effectively tax 20% of their base production in resources. Many NPC's to attack for items, and resources. Advance hero level through battles, and in the long-run barter equipment to improve your troop units strengths.
The best thing about an RTS as such is you can play it as much or as little as you want. Always release new servers so you'll never really be a late starter, always get the chance to re-start when you have a grasp of the game.
If you already happen to play there, I'm Dizzi2004 I tend to be rank 1 on the servers I play - while I play them :P