First of all, it is a brilliant idea to use an artist's license to present this view. I love it!
From gourd level, Bigelow's men would not have been able to see more than a few score of yards in from of them because of the rise of the ground. However, by giving us a view from the second floor window, I think your perspective might be just about right. I can't be sure, as I have never been inside the Trostle house.
One thought for your consideration would be to look at the two men immediately behind the cannon on the right, the one that is firing. I thing they would be further outside the wheels to avoid being injured when the gun recoiled. Maybe you are already planning a change for the final version. (I admit this is rather picky.)