"Here I have some plans for the prototypes of new building levels! If you let me experiment a little, I'll help you to upgrade your buildings. For free! But decide now because I never stay in one place for long."
The architect is an event that appears randomly (every 10 days or so), showing up outside your castle in a camp called the Architect's pavilion. It lasts for a short amount of time (1-12 hours, depending on when you see it).
The highest levels for certain buildings can built only during the architect event, for no cost. These include: