Edupics – The Middle Ages section – dozens of pages are available from this site, including many that show knights, scenes from the Middle Ages, and castles. One of the best images is this 15th century alphabet.
Printable Medieval Coloring Pages from PrintActivities.com – includes pictures of castles, coats of arms, and medieval patterns. They also have a section of other printable activities, such as mazes, word search puzzles and dot-to-dot puzzles.
Fantasy and Medieval Coloring Page Sheets for Kids - from BlueBonkers, this site has various images of knights and castles
Skipton Castle – the official site for 11th century fortress from northern England. Includes detailed printable colouring pages.
Vintage Printable - these images are already fully-colored – they are from medieval manuscripts and artwork, but can be useful to include.