[quote name='The End']So I went ahead and bought it.
I've activated keys for games before, but this is the first time that I've ever tried to activate a game bought at another digital distribution store. If I were to activate this game on Steam, would that prevent me from ever playing this game through Amazon? I probably wouldn't want to ever play it through Amazon, but I'm just curious as to how it works.[/QUOTE]
As far as I know, no, it shouldn't prevent you from downloading/playing from Amazon's game client. Think of getting the key to activate on Steam as a bonus (much like those Humble Indie Bundles that always pops up). Come to think of it, DoWII might be a Steamworks game. If so, it would then require Steam to play.
And in regards to the deal: totally worth it. Even if you have only a slight interest in fantasy-RTS.