Castle has recast the role of NYC Mayor, Robert Weldon. Damages and Harry's Law alum Derek Webster, will take over the role originally played by Joseph C. Phillips in season 1. Early on in 2012, Webster's character will take on a more active role on Castle. Mayor Weldon has an established, long-time friendship with Richard, but he'll take center stage as a high profile scandal surfaces that could threaten his career.
Source: TV Line