Cossacks II is bad. Last add-on for first one is good. Well, one match in Cossacks can last up to 6-7 hours, this is some hardcore strategy with a very close-minded community. I mean that you won't meet a single newbie playing multiplayer, and that makes it even more harder. You can find gameplay @ YouTube. There is even more hardcore strategy from GSC - American Conquest, but it's not played these days (although it's more hardcore than Cossacks, that's why) and matches there can be even longer when played in party of 6-8 professional players.

I remember a good old game called Earth 2150 - who played this too? ^.^

And my second fav, Imperial Glory by Pyro Studios