"Warriors' Gate" was significant in that it was the last story to feature companion Romana played by Lalla Ward as well as the last regular appearance of John Leeson as long-running companion K9. Ward had requested to leave earlier, having been offered a part in another series, but Nathan-Turner kept her to her contract. It was also during the making of this serial that Tom Baker let it be known that he would leave at the end of the series. Ward and Baker were in a relationship and had been for some time, but by now things were turning acrimonious between the two, with many production personnel believing that they were on the point of splitting-up. It was with much surprise when they learned that just a few weeks later they had married. Leeson, who left the series at the end of Season 16, returned for Season 18 on the understanding that K9 would be written out toward the end of the season.