Help with filling in hollow structures


Beginner user here. I have designed an incense holder with a circular-ish base with buildings on top of it. Both the base and buildings are hollow. How can I fill them in to be solid? Some objects go below the surface into the base’s inside, so it has been difficult for me to try and make them solid. I have attached photos of what I am talking about. I can also send the file if that helps.

Hopefully you can see what I am talking about from the pictures I uploaded to the imgur link. There are both images of the outside and then the issue.

Thank you