A relatively simple wargame (no construction or research) with a scenario editor that was surprisingly robust for the time. Define units, draw their map icons, and import .wav files for their movement and "melee" attacks. You couldn't change the terrain graphics, but could define how each terrain affected movement and combat on a per-unit basis. The only thing I wanted for was a concept of "fuel," which limited its use for simulating naval combat.
I spent *thousands* of hours tinkering with this, using it to build entirely fictional campaigns in my personal SF universe. One of my favorite tricks was limiting units in "transit mode" to roads and cities, effectively turning them into railroads for strategic redeployments (which, sadly, the rather basic AI could not handle).