This is the definition of a treaty:
a : an agreement or arrangement made by negotiation: (1) : a contract in writing between two or more political authorities (such as states or sovereigns) formally signed by representatives duly authorized and usually ratified by the lawmaking authority of the state (2) : private treaty
b : a document in which such a contract is set down

When there is negotiation for an agreement or treaty, it is not unilateral, meaning not one sided. By all means, one party may be in a stronger position to negotiate with terms and conditions apparently more favorable, but this does not mean it is unilateral nor does it mean one party dictates all the terms and conditions. If that were the case, then this one party can just dictate, there is no need for an agreement or treaty. This one party might just as well set all the rules and punish those who are not following them, and this is call dictatorship.

You can still have an agreement/treaty and one or both parties might not follow through with it, but that does not mean it is a unilateral agreement/treaty. Both parties did sign and agree to the terms and conditions in the beginning so there is bilateral agreement/treaty. Things can happen during the period of the treaty that lead a party to breach the terms, but that does not mean the agreement/treaty is unilateral. You might not be able to contest it (in court or otherwise) due to circumstances such as financing or other terms, but that does not mean the agreement was made unilaterally by one party alone defining the terms and conditions.

In terms of city agreement in your example, there are agreements such as restrictive covenants and caveats that tie to the land and not the person. The terms of these agreements will be transferred to the new owner as they run with the land. However, usually these agreements (i.e. restrictive covenants) might have term limits, which means the agreement with the city on the covenants may end in 40-50 years then the owner can have that agreement (i.e. restrictive covenant) discharged from the land. These matters are related to municipal laws and property laws.