According to scenario writer Ulf Andersson, the story was set several years after the events of Final Fantasy XII: Revenant Wings.

The plot revolved a magical fortress designed to defend Ivalice from a being known as the Sea King Loemund, who rose every 10,000 years to attempt to conquer the land.

Though the myth is mostly disregarded, the story is believed by Basch fon Ronsenburg, now serving the current Archadian Emperor Larsa Solidor under the name of "Judge Gabranth".

[6] The art style of the game was supposed to be realistic and similar to The Elder Scrolls V: Skyrim according to Guillaumue Mraz, a level designer at Grin.

Square Enix president Yoichi Wada visited Grin several times, and liked the action role-playing game concept and its Nordic visual style.

[2][7] After getting the chance to see a boss fight from Bionic Commando, which at the time was being developed by Grin for the Japanese company Capcom, Wada said he had seen enough, and decided that Square Enix would publish Fortress as a Final Fantasy spin-off.

[4][5][10][11] A portfolio video created by lead technical artist Anders Bodbacka revealed that Larsa Solidor and Basch fon Ronsenburg from Final Fantasy XII were also to be featured.

Planned boss battles included fighting a gargantuan version of the Final Fantasy monster Malboro, where the player would use the seaweed on its back to climb on top of it and drop bombs on the creature's weak spots.

[13] Grin's music director Erik Thunberg was responsible for the game's score, including a track that features a rearrangement of the "Prelude" theme from the Final Fantasy series.

[2] In 2009, Grin released Terminator Salvation, Wanted: Weapons of Fate, and Bionic Commando to negative reviews and poor sales, which caused further financial woes and seemed to make Square Enix nervous.

[7] The developer closed its offices on August 12, stating that delayed payments from "too many publishers" caused "an unbearable cashflow situation" and referred to Fortress in a farewell note as an "unreleased masterpiece that [they] weren't allowed to finish".

The video description stated the game's events are "set some time after Final Fantasy XII: Revenant Wings" and mentioned Square Enix's subsidiary Eidos Montreal as a possible new home for the project.

[22][23] Replying to a fan question in May, David Hoffman, director of business development at the North American branch of Square Enix, mentioned Fortress without confirming its existence, also saying that he had no involvement in the rumored project.