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This page is unfinished or has key details missing! The hard-working cats over at toiletjointpizzaghost.party are working overtime to bring you the highest quality Freedom Planet development info.

Some of the wording/sorting/layout of this page could be better, and some development information from 2012 onwards is missing (mostly because it's really obscure and I also didn't have a Sonic Retro account to see the game's development thread on there (and I also want to finish the site for FP's 12th anniversary (and I'm tired LOL))).
I'll dig out what I can and release an update to the site eventually!

Info

Most of the game's known development details, slightly cut down to be a bit more digestable.

2011

The earliest screenshot of Freedom Planet still available.

Freedom Planet began it's journey as a Sonic the Hedgehog fangame, to beat a dead horse.

"My Sweet Purple" by Ziyo Ling

Sash Lilac, alongside the other members of Team Lilac, were originally Sonic OCs created by Ziyo Ling in the early to mid 2000s.

Sabrina DiDuro (from now on referred to as just Sabrina) had originally started development in late September 2011, in the middle of working on a cancelled game of hers called Spectrum Chase.
The filenames for some early screenshots indicate Freedom Planet was referred to as sltest by her (likely meaning "Sash Lilac Test"), but later it gained its final name as merely just a codename.
Sabrina initially used Lilac without permission, but was allowed to use her for the game a few days later.

Even very early on, a lot of ideas from the finished game were in place, such as plans to add unique powerups and the energy meter to perform special abilities. Moves like the Spindash and Super Peelout present in the engine Freedom Planet was based on, Sonic Worlds, were stripped from it and there was no intent to include Sonic characters of any sort.

Still, the game was intended to be a Sonic fangame even without the blue blur's presence. Rings were originally used to shield the player from damage (you had a health bar and a ring bar/"Bank"), though rings were later turned into your only form of health.

Super forms were also briefly considered, though ultimately were never implemented:

An early sprite of Dr. Brevon, next to Lord Brevon.

Brevon was also a pretty early mainstay of the game. Before donning his title of Lord, he was moreso a modest Doctor. As modest as an alien that wants to drain your planet's energy source to power his ship can be, I guess...
He originally began life as a Neopets OC, based on an old character from the game called Dr. Sloth. Due to Viacom buying out Neopets, an intended return of Dr. Sloth was cancelled. This led Sabrina to create Brevon in "2007-ish" out of frustration.

An early sprite of Spade.

Spade was also made pretty early on and was a cavy instead of a panda. He was intended to perhaps be Brevon's sidekick, and his attack patterns were planned to be WAY different than his final counterpart.

Early versions of some characters.

Serpentine and Torque were also already around.

By October 22nd, there was enough of the game in place to post footage of it. It is unfortunately lost, though.

People in the development thread were seriously impressed by the quality on display, and Sabrina was proud of her work so far.

Early versions of some other characters.

A couple more concepts and level designing sessions throughout November and December later...
And the infamous December 14th 2011 footage of Freedom Planet was shown off!

Screenshot from the footage.

It still bars a lot of things from Sonic, such as rings and some custom spring designs, but the game still looked quite impressive to forum members.

The gang's now together.

Relic Maze 1 was also being worked on when this footage was posted, and Sabrina was also given permission to use the other two Team Lilac members by this point.
Additionally, Leilani R. Wilson (from now on just Leila) and Claire Ellis (from now on just Claire) were hired as the game's composers shortly after!

2012

Development this year was mostly focused on adding more stages, while also trying to distance the game away from its Sonic fangame roots.
Battle Glacier and Thermal Base were mentioned as planned levels by this point, though interestingly both stages were supposed to have water in them.

Early preview of the Petals.

Early into the year, Relic Maze was shown off alongside the replacement for Rings, Life Petals. Feathers were also considered as a replacement for them.

Early shields.

Shields were implemented pretty early on in the year too. Early ideas for each of their abilities were shared on SoaHCity's "Post your screenshots thread", though most of these were reworked:

Early preview of Lilac's Treehouse.

Lilac's Treehouse was also being worked on! At this point in development, since hub areas were still in the plans, you were meant to use Crystals to buy furniture for the treehouse (with different palettes available for each piece of furniture!).
Another idea for furniture was a jukebox that would let you change the background music of the area, though both this furniture customisation system and hub areas were cut in the final game.

The game was also submitted to the Copenhagen Game Collective's Nordic Game Indie Night (NGIN) in February, though it was rejected.

A special build of the game, most likely containing Dragon Valley, Relic Maze and Lilac's Treehouse, was sent to a few SoaHCity members for testing before being sent for submission to the event. The build is completely lost, with the best lead for its location being a landfill (and no, this is somehow not a joke).

Screenshot of Relic Maze footage from April 2012.

More footage of Relic Maze was shown off in April, also showcasing the old shield system featured in early demos.

In late April, Sabrina opened voice acting auditions for various Freedom Planet characters. In these audition reels, there are early lines from the script alongside early character descriptions and sprites:
If you wanna skip past these (there's a lot), click here.

*recreation of the image used since it's lost (it probably looks like this though)

:: Sash Lilac

Determined - Compassionate - Clever

The heroine of the story. Lilac hopes that through kindness and compassion, she can make up for the mistakes she made when she was younger, though her desire for inner peace causes her to act irrationally at times. She ends up causing trouble with one of Dr. Brevon's henchman, and from that moment forward she's thrust into the struggle against him. Because of her attractive looks, she's initially mistaken for a princess, and she uses this confusion to her advantage to get where she needs to go.

:: Carol Tea

Reckless - Adventurous - Loyal

An energetic wildcat with a thirst for adventure. Carol grew rebellious towards her family, and she eventually left them in order to search for her estranged sister. She met Lilac along the way and the two of them made an instant connection. Carol is impulsive and tends to get in trouble, but she's intensely loyal to anyone she considers a friend.

:: Milla the Hound

Curious - Shy - Friendly

Milla can track sources of energy with her large ears and keen sense of smell. She has many dog-like instincts and will often do things like stick her tongue out, chase shiny objects, and run on all fours. She's bashful, but when she opens up she reveals her down-to-earth nature and colorful imagination.

:: Serpentine

Sharp - Deceitful - Insecure

Serpentine has a great sense of direction and often goes on scouting and recon missions. In exchange for his service to Dr. Brevon, he was given a pair of robotic arms so that he could perform actions that other snakes aren't able to do.

*may not be the exact sprite used, could be a derivative of it

:: Spade the Cavy

Trickster - Reserved - Stubborn

Spade draws his power from a mysterious deck of playing cards, which grant him different abilities depending on the order he uses them in. Through the course of the game, he becomes Sash Lilac's rival and seeks payback against her for something she did many years ago...

:: Dr. Arktivus Brevon

Clever - Aggressive - Manipulative

The main villain of the game. Dr. Brevon is a sadistic warlord from beyond the stars who is perfectly willing to drain the life out of Avalice in order to continue his dreams of galactic conquest. He has a very commanding presence, and the other characters' fear of him is one of his defining traits.

:: Torque the Shellduck

Torque is a courier from Shang Mu who was hired to transport secret packages between other cities. He is desperate to expose Dr. Brevon's atrocities, but because he is not a skilled fighter, he needs the help of Lilac and friends to complete his mission. Aside from being a little eccentric, he is a skilled pilot and craftsman, and he has the ability to forge various items from the crystals that Lilac collects throughout her adventure.

Gender: Male
Voice Type: Medium pitch, at least 18 years - no set voice type, but try and be creative

Audition Lines
1. "Sooner or later, there's gonna be war, and those oversized tin cans ain't gonna care if we're ready or not."
2. (Caught by surprise) "It's not what it looks like!"
3. (Determined) "I'm on my way! Hold on!"

:: The Royal Magister

The Magister acts as the spiritual leader of Shang Tu and the commander of the royal guard. Despite his pride and wisdom, he doesn't have much experience with armed conflicts, and he is unable to stop Dr. Brevon from exploiting the city's resources. As an individual, he is cold and deceitful towards outsiders, but loyal to his people.

Gender: Male
Voice Type: Slightly aged but firm

Audition Lines
1. (Angry) "We will deliver swift justice to our enemies!"
2. "You would be wise not to question my judgement."
3. (Solemn) "We are aware of the invader's true intentions... but we are also aware of the terrible power he possesses."

:: Mayor Zao

Mayor Zao is the leader of Shang Tu's rival city, Shang Mu. In contrast to his rival, who is pacifist and dignified, the Mayor is more bumbling and militaristic. He was appointed several days before the events of the story, when Brevan began to demand enormous amounts of crystal energy from him. With the city's power supply dwindling, he has grown desperate to find a way out of the crisis.

Gender: Male
Voice Type: Boisterous, quick, almost comedic

Audition Lines
1. (Angry)"You see the hat?! I'm in charge here!"
2. (Nervous) "We've been totally honest with you, Doc! Our supplies are dwindling! We barely have enough crystal energy to get through the month!"
3. "Those stones may be our only hope, and those purple-eyed duck monkeys are too stupid to realize it!"

:: Neera Li

Manipulative - Spiritual - Stubborn

Neera believes wholeheartedly that Dr. Brevon's arrivial coincides with an ancient prophecy. As such, she offered her services to him without question. In spite of her docile appearance (which she uses to her advantage), she is harsh and uncompromising, and she often argues with people who don't share her beliefs, including her own partners.

Gender: Female
Voice Type: Confident, at least 20 years old, medium to deep pitch, eccentric

Audition Lines
1. "There are forces at work beyond our understanding."
2. (Threatening) "We don't have time for your questions. If I were you, I would back away very slowly."
3. (Laughs) "You fight like a little girl!"

Early attempts at redesigning Lilac.

Multiple attempts were also made to change Lilac to make her not resemble Sonic The Hedgehog as much. None of these redesigns were liked unanimously, hence them never being used.

A bunch of characters!!

Other characters were also made around this time, some more rough compared to their final counterparts or not going used entirely.

Demo 1.0 screenshot.

In early August, the game finally had a demo release via SAGE! Beating a time record would unlock Carol, and collecting all 10 cards would grant you with access to Mahjong (which was a last-minute addition!).