Cut topography with walls

I have this project in Revit and I want to cut the topography with the 4 surrounding walls of the building:

I’m new to Dynamo, so I don’t know what the best approach would be, but the result I am looking for is getting only the topography inside the walls so I can compare the total area with the constructed area.

So far I have converted the topography to mesh, but I don’t know where to go from here.

Instead of re-inventing the wheel, why not use the built-in Revit tool to create a subregion?