Formal agreements between cities can be made in one of two ways. The first of these is through use of the Treaties BB. This is often the case in the aftermath of war where the details of such agreements become a matter of public knowledge; the specifics important enough that the world at large should be made aware of its details.
The second method in forming such an agreement is using the gold-gilted parchment. These scrolls are not a matter of public interest and it is entirely at the discretion of the involved parties whether to divulge its contents.
Takes you into an editor where you are able to scribe the terms of your treaty using the exact same methods as the Bulletin Board or letter-writing.
Signs the specific scroll on behalf of your city, imbuing a unique codeword into the parchment to ensure forgery is impossible. All involved parties must ratify the treaty for it to become binding, most commonly the Prince or Princess of each involved city and (in the case of military agreements), the Field Marshall.
Note that while it is possible and often advised to seek divine approval for your treaty, the deities are NOT obliged to arbitrate mortal affairs and to make such an assumption can prove a costly mistake indeed...