Heraldic Symbols: https://www.heraldryandcrests.com/pages/heraldic-symbolism-a-z
Glossary of Castle Terms for your Castle Builder project: http://www.castlesontheweb.com/glossary.html
10 Things You May Not Know About the Vikings: http://www.history.com/news/history-lists/10-things-you-may-not-know-about-the-vikings
Building a Medieval Castle in Modern Times: https://www.youtube.com/watch?v=CboJzrDhoSk
Below are some helpful websites for building catapults. If you find any other good ones, let me know!
Various designs:
http://www.stormthecastle.com/catapult/index.htm (no mouse trap catapults!)
Pencil and rubber band catapult: http://www.instructables.com/id/office-supply-catapult/
Relatively simple:
http://www.stormthecastle.com/catapult/how-to-build-a-catapult.htm
http://www.stormthecastle.com/catapult/popsiclestick-catapult.htm
Viking History: Myths and Facts: https://www.livescience.com/32087-viking-history-facts-myths.html
Horrible Histories: Vicious Vikings: https://www.youtube.com/watch?v=WIwFJPwdxno
Medieval Weapons: http://www.lordsandladies.org/middle-ages-weapons.htm