Directional Light Shining Through Single Sided Geometry

When I bring a model of a building facade into a standard default scene with the one directional light, the light passes through the back of the mesh as if nothing is there. I was expecting the light to interact with the geometry despite there being back faces but that doesn't seem to be the case.

Does anyone know how I can fix this?

On the left I have the inside of this castle forum type area and on the right I'm showing it from behind the facade. You can see how the light is only interacting with geometry that faces it instead of everything.

Any help is appreciated :)

I'm using Unity 5.6.3p1