1. We can add colour border around button (in this example is white colour around Karikala Chola hero)
http://imgur.com/bO1DnAO
2. We can add circle in the same colour as a frame around button
http://imgur.com/UTDqH3d
I like the circle better for heroes as they are a very different category of units, a strong differentiation is desserved. But the idea of the colour border would be good if you intend to further differentiate units categories (such as civilian/siege/etc.); I'd suggest increasing the border width a bit to make it more visible though.