Tonight on The Flash, the gorillas of Earth-2's Gorilla City will wage war on Central City in the second half of an epic battle between The Flash and Grodd, which began in last week's episode.

But it's been in the works for much longer than that, according to showrunner Aaron Helbing, who told ComicBook.com that the writing staff had considered introducing Solovar, the monarch of Gorilla City, as early as last season.

"Grodd plus The Flash plus Solovar plus the arena. That ws kind of it," Helbing said in an interview. "To be able to have Grodd and Solovar against The Flash, to see Flash in that arena in Gorilla City, which we teased last season when we sent Grodd there. That set up exactly where we wanted to go and what we wanted to do with these characters."

Helbing said that while last season provided the setup, it was only the vague details that were in place before they decided to make this year's Grodd story the two-part "movie" of the season.

"Broad strokes. Nothing in too intimate detail, but we knew that we wanted to take Barry to Gorilla City," he said. "When we took him over toe Earth-2 last season, we loved Earth-2, we loved Grodd, and we talked about introducing Solovar last season but it just didn't work emotionally and story-wise. We thought it was a cool aspect, to explore with Grodd went and what happened to him. He's ascended to be Solovar's number two, but he has plans of his own because of being betrayed by all the people he thought were close to him."

Audiences got a sense for those plans last week, when Grodd manipulated The Flash into defeating Solovar so that he could take over the army and wage war on Earth-1. The Flash thought they were safe, but this week's episode sees Grodd and company escorted to The Flash's world and ready to wreak havoc.

MORE: New Poster Teases Gorilla Grodd, The Flash Showdown | Comparing Gorilla City to Its Comics Counterpart | Recap With Spoilers: Attack on Gorilla City

CW's The FlashWednesday at 8PM on CW

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Based on the characters from DC, The Flash is from by Bonanza Productions Inc. in association with Berlanti Productions and Warner Bros. Television, with executive producers Greg Berlanti (Arrow, Supergirl), Andrew Kreisberg (Arrow, The Flash), Sarah Schechter (Arrow, DC's Legends of Tomorrow) and Aaron and Todd Helbing.

The Flash airs Tuesday nights at 8 p.m. ET/PT on The CW.