Video Footage used in still photo composition
I have a question about using video footage on a still photo background. The general idea is that I have a picture of a field with a river in front of it. Placed on the field is a trading fort (which is video footage filmed on a green screen). In the video footage, the camera angle goes from a flat front view of the fort (which appears as expected on the photo background). As the camera angle goes from flat front to a bird's eye view inside the fort, the fort appears to "sink" into the ground in the still photo background.
My question is, is there any way to keep the fort ON the ground instead of it appearing to sink INTO the ground?