I’m doing research on Medieval Britain (esp. between 900 A.D. - 1400 A.D.) for my writing. But I'm having difficulty finding information on weapons and military strategies of the time.

Could anyone please recommend some good websites or books on this?

Try these sites for links to the info you want:

http://www.backtowhen.com/pages/contacts/historical_links/historical_links.html

http://www.dc.vccs.edu/lrc2/resources/historyweb/medieval%20.htm

http://www.sd68.bc.ca/dove/library/medieval.htm

Thank you, those links should be very helpful.