You'll not find too many that are still played online, though I believe Netstorm still has a nice following at
http://www.netstormhq.com - very old game, tons of fun once you get used to it, and the campaign has a difficulty curve like EVE's learning curve... that is to say very steep. Games do tend to go on for a while if you get a stalemate going on though.
If you're looking for something slightly newer, Warcraft III's a good bet, as are most of the Command & Conquer games. Dawn of War's awesome too, very awesome.
I know you said you don't want to advance ages and such, but Age of Empires II is extremely good, still one of my fav RTS games. Especially if play is limited to the Castle Age, Imperial Age is a little too overpowered so you can't really get a good defensive style of play going on since Trebuchets outrange most of your units and you have to keep a strike squad of treb-killers just inside your gates and even then if the trebs are defended then they might die leaving you vulnerable, and eventually dead.