You can order both "Spartan" and its sequel "Troy" (not "Trojan" - wouldn't be a good name for a downloadable game, would it?
) from
the developer and publisher Slitherine Strategie Software UK Ltd. either as boxed copy or as download. You can order it via download from matrixgames as well, btw.
All campaigns from "Spartan" are auto-importet into "Troy" upon installation, that way you can play all the campaigns with the lastest features of the "Troy" release.
For both games there are 100Turn-Demos available, which allow access to a tutorial scen and 1 small and big map each IIRC. Sadly, it looks like the map editor is not included in the demo
Have played the demos of both games and its predecessor "Chariots of War" for quite some hours, but never got around to buy the games because I don't have a credit card ...