Just copy warcraft 3. Unironically, unapologetically, the retards at blizzard can't seem to understand what the appeal of warcraft 3 was anymore, so just use that. Well, make sure to keep it legally distinct, but that's exactly what I'd go for if I made a tower defense or rts style bird-eye view game. Alternatively, pixel arts, but yeah, models aren't nearly good enough to be reliable on that front, even for unimportant unit works like this.