Stana Katic did not leave "Castle" in 2015. In May 2015, she renewed her contract with ABC for the show's eighth season. Her agreement with the network was up at the end of season seven.
Katic's co-star, Nathan Fillion, signed on for the eighth season in April 2015. There were some exits from the show, however. ABC did not keep Penny Johnson Jerald, who plays Captain Victoria Gates, in the cast for the eighth season. In addition, Terence Paul Winter and Alexi Hawley take over for the season seven showrunner, David Amann. Both Winter and Hawley have worked as producers on "Castle."