The Starbucks example illustrates the two main considerations of treaty design:
Workflow. How does work flow from one fortress to another?
Error management. How does one fortress report an error to another fortress?
You might expect security to be part of the treaty. In the Starbucks/Roger collaboration, I had to hand over my American Express card to prove I was authorized to make a charge on that particular account. It is reasonable to think of security as part of the treaty, but I think of it as a feature of the drawbridge rather than of the treaty, and I recommend documenting the drawbridge in the fortress documentation. Either approach is fine, though.
Treaties are spelled out in treaty overview documents (TODs), ...