Paul Morrow was in the original script for this story but Prentis Hancock's morning makeup man noticed a lump on his throat; Prentis ended up at the hospital for an operation so his lines were given to a new character Winters (Gary Waldhorn).
Gwent was built in two scales (one five feet wide and the other fourteen inches, plus larger sections for the lift and Eagle docking). The larger model was smashed by Nick Allder immediately after filming, in frustration after problems making the ship roll correctly.
The tail end of Year One had been running out of steam budget-wise, so head of ITC Lew Grade put up some more cash, retaining the show's high quality (particularly noticeable with this episode, Dragon's Domain (1975) and Mission of the Darians (1975)).
Two Gwents - as well as three laser-equipped tanks - were built to scale with a 44" Eagle by Martin Bower in 12 days.
Filmed December 20th 1974 through January 9th 1975 (20th episode in production order), taking breaks for both Christmas and New Years.