How to render a custom shadow map in 2D game?,How to render a custom shadow map in a 2D game ?

Hi everyone. I am developing a 2D game with 45 degrees view but I want to use 3D real-time light in it. And I have a problem with the shadow.

Because it is a 2D game, unity seem like have no way to cast the shadow，so I think maybe I should do this in myself.

My idea is that get the height information from a height map, so I can get the 3D coordinate. And then I will get the depth map in the view of light. The question is that how I can render the depth into a texutre?

Or is there some methods else to solve 3D real-time light in 2D game?

Thank you very much! And Please forgive my poor English.