a. Use windows from dolls house suppliers (these can be rather large)
b. Use windows from model railroad accessory suppliers (O guage and 00 guage are available)
For both of these you would need some sort of picture to see through the window and appropriate glazing (optional).
For the palace hall I just printed some sky on gloss photo paper, cut it to the appropriate size, made frames of cardboard and indiciated glazing bars with a black ink pen.
Materials - I use mostly paper and cardboard, and the approriate glue. Its cheap and I find it more flexible.
For the library I wanted to add 'patio doors', to act as a picture window for the King to sit at his desk and look out. Firstly I cut out a template, then made the door frames; I selected a suitable picture and printed it out and cut a clear piece of plastic to use as the glazing (you could print the phot on gloss paper if instead). All the bits ...
After assembling the basic structure I added the doors, I did this by cutting strips of inkjet labels and sticking them on - wide strips for the door edges and thin strips for the glazing bars.
After that the 'patio doors' are fixed to their position in the room. It seems the King has come to check the builders have fitted them properly and approves of the result....
The principle of constructing the patio doors can be applied to any window. Next up pictures and furniture.