If it was from 10-20 years ago, top down from an angle with modeled 3d units, it might be one of the Wargame titles from Eugen, or if it was WW2 setting maybe Combat Mission: beyond overlord, Company of Heroes or Men of War.

If it was straight on top down 2D, it might have been Mud and Blood, which was a WW2 wave defense flash game.

Was it top down in the sense of looking straight down or from above at an angle? Were the units modeled as individual 3d models or just 2D icons?

Also, roughly what time period was it set in? Like, Napoleonic, WW2, Cold War, Contemporary?

Was it single player or multi player focused?

Could you get additional units as the game went on or were you locked with the units you started with? How could you get additional units? Points? Timer?