This post contains spoilers for Arrow Season 5, episode 8, titled "Invasion!"

In the Arrowinstallment of this week's CW superhero crossover, Oliver Queen (Stephen Amell), found himself abducted by the Dominators, along with his allies Thea (Willa Holland), Diggle (David Ramsey), Ray (Brandon Routh) and Sara (Caity Lotz).

While captured, the aliens placed the quintet in a shared dream that offered them a seemingly idyllic existence: Oliver and Thea's parents, Robert (Jamey Sheridan) and Moira (Susanna Thompson) were alive and well, as was Sara's sister, Laurel, whom Oliver was engaged to marry.

The trip down memory lane was designed as a way to pay homage to Arrow's history during the Invasion storyline, since the episode was also the show's 100th.

Arrowshowrunner Marc Guggenheim admitted that it was tough to find the balance between servicing the 100th episode and the crossover storyline, telling reporters at a recent press conference, "I think it’s pretty obvious that we chose to go 100th episode, but it was totally a challenge. It was Greg [Berlanti]’s idea to have our five Arrowpeople and basically enter them into -- we call it the matrix -- the Dominator matrix, and that allowed us to have our cake and eat it too."

If the scenario seemed reminiscent of The Flash's Flashpoint storyline or last season's Supergirl episode "For The Girl who has Everything" -- both of which saw Barry (Grant Gustin) and Kara (Melissa Benoist) experiencing an idealized version of their lives where their own parents were still alive -- that's by design, per Guggenheim.

"There’s a reason why these stories are iconic or familiar tropes, even in comic books. It’s because when you show the protagonist the path not taken and you basically put them in a situation where they can choose to stay on that path or go back to their life with all of its ugly aspects and challenges... when they choose the selfless choice of returning to that ugly past, it makes your character stronger," he pointed out. "It forces Oliver to double down on his mission and commit to this life, with all of its losses and failures and challenges."

In honor of the 100th episode, Arrowattempted to bring back as many former cast members as it could, including Colton Haynes (who plays Roy Harper) and Colin Donnell (who played Oliver's best friend, Tommy Merlyn) -- but scheduling conflicts prevented them from returning.

Tommy's absence was explained with a meta wink in the episode when his father, Malcolm (John Barrowman) mentioned that Tommy was working at a hospital in Chicago and couldn't get away (Donnell is now on NBC's Chicago Med).

And, thanks to the magic of visual effects, the show did manage to include both Donnell and Haynes in at least one scene before Oliver left his fantasy life, in which Oliver's loved ones offered words of encouragement to send him back on his hero's journey guilt-free.

"That beat was not in the original break of the episode. Honestly, I can’t even tell you where that idea came from. I was writing that scene where Oliver goes back and it just popped into my head that basically all the actors that we can’t get -- at the time I didn’t know who we could get and who we couldn’t -- I’m like 'this is a way we can get them,'" Guggenheim explained. "And props to our visual effects house, Zoic, [who] handled those shots. They did an amazing, amazing job, particularly with Colin and with Colton, because we couldn't reshoot them. We had to take them from old episodes and roto them out and put them into this. It was hard, obviously, because they had to work with pre-existing footage."

The producers were particularly excited that Cassidy was available to reprise her role as Laurel Lance, executive producer Andrew Kreisberg noted, "because she is such an essential figure to the show. She was the second lead on the show. Even though the show has evolved, Laurel is at the heart of it. She was Oliver's great love, she's Sara's sister, she's Lance's daughter ... even if we do get to 200 episodes, Laurel will always be at the heart of the series and be such an important character. And, on top of that, Katie Cassidy will always be so important to us. We were so happy that she agreed to come back because she is part of the family, both behind the camera and in front of it."

The only conspicuous absence in episode 508, at least from a mythology standpoint, was Evelyn Sharp, aka Artemis (Madison McLaughlin), who was revealed to be in league with Prometheus in episode 507, but remained offscreen when the rest of Team Arrow partnered up with The Flash and Supergirl in this week's episode.

"To be honest with you, in the break of 508, we talked a lot about 'do we have her in there,' and it felt like it was the elephant in the room and we didn’t want it distracting," Guggenheim admitted. "So we don’t reference it in large part because I’m not a fan of, ‘Oh, it’s too bad that Evelyn’s mom is sick this week!’ I would prefer to just whistle past the graveyard. I will say, you will get a payoff to 507’s cliffhanger with respect to Evelyn big time in 509."

Arrow airs Wednesdays at 8 p.m. on The CW. The Invasion crossover concludes Thursday night with DC's Legends of Tomorrowat 8 p.m.