Empire II : The Art of War [http://www.mobygames.com/game/empire-ii-the-art-of-war] by Mark Lewis Baldwin and Bob Rakosky.
It was castigated by for not being like its predecessor (a classic start-with-one-city-and-conquer-the-world game). When I bought E2 in 1996, I was entirely unaware of its predecessor, and found a charming, simple tactical wargame contruction set. I could make my own sounds, paint my own icons, define my own units and layout and label my own maps.
I spent hundreds of hours playing and modding E2 - possibly more than I spent playing and modding Civ2. I recall one guy made a scenario for it covering the German invasion of France and the Low Countries. It had over 400 units and took days to finish.