I'll be turning the Powderhouse into a camera obscura. The principle of the camera obscura (Latin for "dark room") has been known since ancient times, and you can imagine it being discovered by happy accident, as it's really very simple. When bright light (like from outside) goes through a small hole into a dark space (like inside), an inverted image of the scene in the brightly lit area is projected at the point where the light stops in the dark space (like, on the wall). The smaller the hole, the better defined (but dimmer) the image will be.
In fact, all "real" cameras work on this same principle. The lens is the hole, and the film (or digital sensor) is the back wall onto which the inverted image is projected. Usually, a mirror inside the camera flips the image so that it reads the right way in the final photo produced.
The reason this all works has to do with the physics of light. Light cannot bend and wants, always, to travel in a straight line. As Alfred Daniell explains in The Principles of Physics: "When light radiating from an extended object passes through a small aperture the waves arriving at the aperture from the object traverse the aperture and there cross each other they then diverge and a screen placed on the opposite side of the aperture receives an inverted image of the object whose size varies with the distance of the screen."
If you're interested in the nitty gritty of how the physics work, this book (available free on Google Books) will give you lots of details and lots of lovely flowery prose: Elements of Physics by Arnott and Hays.
This is how it will work at the Powderhouse: I'll darken the window and the door with cloth or plastic, blocking almost all the light in the structure. There will be a small hole in the covering over the door. Across from the door, I'll hang white sheets on the wall. Images of whatever's outside the Powderhouse, on the hole side, will project onto the sheets. I'll capture these projected images with a digital camera (and maybe also a film camera).
In theory, it would be possible to use a light-sensitive surface, such as photo paper or film in lieu of the white sheets, and make the images directly with the Powderhouse. I've opted to go this route, of capturing the projected images with a second camera because of the scale of the project and the fact that I want to make lots of pictures quickly (and within the constraints of a small budget).
You can replicate this project fairly easily at home. A bathroom, or another space with at least one, but not too many, windows is a good place to try it. You can use a calculator intended to help with the construction of pinhole cameras to help you determine the best size for your pinhole (optimal diameter) based on the distance (focal length) from the hole to the surface you'll be projecting on (usually the back wall of the room). You'll be surprised how small the hole can be. Usually you'll need to create it using a thumbtack or needle while measuring with a ruler under a magnifying glass or loupe. If you try this, let me know how it goes, and have fun!