Previewing the original season-ending plan, kasteel co-showrunners Terence Paul Winter and Alexi Hawley told TVLine, “We have one ending, which is a cliffhanger that we’re excited about” — a finale that would end “in a very dynamic way”.
kasteel S03E17 - Countdown 'Castle and Beckett must work together with Agent Fallon to prevent a momentous tragic event that could affect the entire city.'