In the opening segment, in regard to the enemy vessels, Spock declares "Romulans now using Klingon design!" The actual reason for this was that in unpacking the models to shoot this sequence, a production assistant stepped on and broke the Romulan Warbird model that was going to be used, so they pulled out their Klingon battle cruiser model and wrote the line to cover it up.
First broadcast episode of Star Trek (1966): The Original Series to feature the D7 Klingon battle cruisers. Although the episode Star Trek: Elaan of Troyius (1968) was produced three months before this episode, and technically the first to feature the D7s, this episode was aired on television first, since NBC changed the airing order for all the episodes. For the Remastered series in 2006, digital shots of the D7s were inserted into scenes in the episode Star Trek: Errand of Mercy (1967), which now officially makes that episode to be the first to have the D7s.
Although long thought otherwise, from the very first draft, the script had the Romulans using Klingon ships. The series had a lot of money invested in the Klingon model and needed to get its money's worth.
The Matt Jefferies-designed Romulan symbol, composed of a yellow hexagon in the center with three colored spokes coming out of it, can be seen outside the commander's quarters, above her door. This symbol never appears again in any Star Trek series or movie.
Joanne Linville's Romulan Commander is given the name Dion Charvon in a 1977 Star Trek novel called "The Price of the Phoenix", by Myrna Culbreath and Sondra Marshak. In the 1999 novel "Vulcan's Heart" by Josepha Sherman, her name is given as Liviana Charvanek. It's not clear if either novel is "canonical" but Liviana Charvanek seems to have higher authority and credibility. In addition, Diane Duane's 1984 novel "My Enemy, My Ally" states that the commander had her name stripped from her because of the events of this episode, and introduces the commander's aunt, Ael t'Rllaillieu.
D.C. Fontana's initial inspiration for this story and its title was the Pueblo incident which involved the capture of an American patrol boat, the USS Pueblo (AGER-2), by North Korean forces during the Vietnam War. The incident occurred on January 23, 1968, just two months before Fontana completed her first draft story outline.
In this episode Spock scans space out to one-half parsec and says that there are no ships present. As the Enterprise enters the Neutral Zone Romulan ships appear from nowhere. Later Spock says that the Romulans must have invented a cloaking device but they already knew the Romulans had one as they witnessed it in use in Star Trek: Balance of Terror (1966).
In Star Trek: Balance of Terror (1966), the Romulan ship only had simple impulse engines. In this episode, the Romulan ship is capable of going faster than Warp 9 - a major upgrade in capability for such a short time.
If you look closely, actress Joanne Linville allowed her eyebrows to be shaven for this character. Actress Jan Sterling in the early '50's did the same thing, and they never grew back - she pencilled them in for the rest of her career. Look at any successive actors playing Klingons, Romulans, and many other aliens for the rest of any Star Trek series, and you'll see that the makeup usually incorporates their natural eyebrows into the character's makeup.
In the episode Buffy the Vampire Slayer: Homecoming (1998), Cordelia claims to be proficient in the Vulcan death grip; an obvious lie because as this episode proves, it doesn't exist and was just something Spock invented to fool the Romulans.
The trivia item below may give away important plot points.
Once again Nomad from "The Changling" has been repurposed to now be the Romulon cloaking device. Only the head is used. The three antennae have been modified slightly to put blinking lights around them.