A number of issues factored into the decision to cancel the series, one of the biggest being the negative reaction of the show's fans to the announcement in April 2016 that ABC was not planning to renew Seamus Dever and Tamala Jones's contracts for a ninth season.
Aired after the announcement of the series' cancellation. In an amusing irony, it ended up being the season's highest rated episode and likely would have helped the chances for the series' renewal, even with ABC's controversial decision to let Stana Katic go.
Rob Bowman, the director of this series finale, was also the director of the very first episode of the show.
DYE-2 is a real but abandoned early warning radar station. It is in Southern Greenland. There are 4 DYE Radar Stations. They are called DEW Line which stands for Distant Early Warning Line. Used by the United States during the Cold War as an early warning missile detection system. They have been abandoned for many years.
TV shows often try to find some element to bookend a series, i.e., reusing something from the first one and use it again in the last episode. Given the way this show ended expectantly they had to improvise. They simply reused Castle's flirtatious double-entendre ("We could debrief each other") from the first episode during the voice over at the end of the last one.