We have a new term for treaties to make them somewhat more binding then simply good word.

PENALTY <gold amount> : (Between cities only) Demand a penalty
when is a treaty is voided. Once the
treaty is signed, the gold will be taken
as a bond by the Iron Bank from the
Ambassador. If the treaty is voided,
then the offending party will need to
pay out the penalty. Mutually dissolving
the treaty will avoid the penalty.

You can reference this in HELP TREATY along with the rest of the treaty actions.

Enjoy!

Penned by my hand on Kinsday, the 11th of Midsummer, in the year 504 MA.