The fourth season of the ABC show concluded in the US last night (May 7) with Castle (Nathan Fillion) and Beckett (Stana Katic) finally getting together.
Explaining the decision to have the characters start a romance, Marlowe said that he felt now was time to "confront" the tension between Castle and Beckett before it became "artificial".
"Ultimately, you get to a fork in the road. If we kept them apart any longer, say another season, in my mind that would have been the death of the series," he told TV Guide.
"Then, we would have just been being artificial. They would have become brother and sister, and then we're just ignoring everything that had happened.
"There comes a point where you have to confront it. We wanted to get the point where we resolved it and made a promise for next season that these two are going to have to be dealing with their feelings for one another."
However, Marlowe was reluctant to define Castle and Beckett as a couple, teasing: "I think those are all the questions the characters are going to be asking themselves, and that's going to be part of the fun.
"When they wake up the next morning, are both of them still in the same place? And Beckett's actually resigned from the force. So, what is she going to do with her life now? There's all sorts of stuff to jump into next season."
In a separate interview, Marlowe admitted that writers had come "dangerously close to trying the audience's patience" with the Castle flirtation.