You must find all 33 Teutonic Flags throughout Acre.
for an awesome guide to the Teutonic flags (as well as all other flags) and the Templars.
Please give full credit to FiReStOrM^
for his awesome work and dedication to making this incredible resource.
In case his page goes down again, I have left videos of all Teutonic flags in Acre. Credit goes to youtube user TheGameStory