Cossacks: Back to War – it's not difficult game but it's classic
YES! I bought the "Kolekcja Klasyki" edition of Cossacks last time I was in Poland. It has the Cossacks: Euro Wars, Art of War and Back to War. Awesome stuff.
You can even play as Switzerland and build up a fierce navy. You know? SWISS NAVY – ruler of the seas.
However, my fave RTS of all time would have to be:
I downloaded a ton of custom units and it is just awesome. A few years ago, a fan made version called "TA Spring" was released, but the official sequel is "Supreme Commander" but I never had a good enough system to run that game. The classic TA just rocks, but it was overshadowed by StarCraft in the games market, though.