Originally as an idea for a Disneyland Paris attraction, the concept for a ‘free-fall drop’ attraction began in the 1980s. Part of the original plans for Disneyland Paris was Geyser Mountain in Frontierland - just past Phantom Manor, next to the geyser that exists today. This would have been a mine train ride through a mountain, with a geyser bursting out from the tracks and catching your train from below. then pushing it up a chasm and out of the top of the mountain - and dropping you back down onto the tracks. The water jets would have hidden an elaborate free fall mechanism. This attraction never got off the planning stage, but the idea stayed in Paris. Discovery Mountain was to be built soon after DLP opened in April 1992, being twice as large as the present Space Mountain. As well as housing the space rollercoaster, there was to be a Journey to the Center of the Earth attraction and had a freefall ride vehicle. At least 2 versions of this were looked at - one had a vehicle with a `drill` underneath it to plunge straight down into the floor (volcano), the other was a more elaborate steel rig where the ride would either start or finish with the passenger vehicle on its side in a curved spur off of the main drop shaft. Due to DLPs financial issues, Discovery Mountain was shrunk into Space Mountain, and so the idea was put away again.
Once the Disneyland Paris ideas were scrapped, the Walt Disney Company was looking to expand Disney-MGM Studios, which was, at the time, a small park with shows, and three attractions. The expansion would involve a new ‘land’ area coming off of Hollywood Boulevard, Sunset Boulevard. All early plans for the land were scrapped, from a Mickey Movieland, to a Roger Rabbit attraction with licensing issues, and Dick Tracey film struck out in theaters. What was needed was a major E Ticket Attraction, and they preferred that it would act as a weenie for the new land, to move guests past and thru the shops of Sunset. Space was reserved for future attractions on Sunset (Rock 'n Roller Coaster`s plot, Fantasmic!'s plot, the third plot between these two and the Farmers Market area - always planned to be a temporary structure.) Until these spaces were filled, the Disney-MGM Studios needed a must see at the end of Sunset. The Imagineers dug out the free fall attraction plans.
In the fall of 1989, Mel Brooks, Michael Eisner, Marty Skylar, C. McNair Wilson and the rest of the Imagineering group met together. It was the beginning of an attempt by Eisner to bring Mel Brooks over to Disney to produce his films at the then brand new Disney-MGM Studios. Mel initially had to be sold on the idea of a theme park attraction after being explained that a theme park attraction has a lasting effect of being in place and seen every day of the week by about 20,000 people for anywhere from 10-20 years. Mel Brooks made about six trips to Imagineering and a number of telephone calls to work on the attraction. Disney Imagineers wanted to shoot for something scary and funny with Mel Brooks and at one point in the early development with Mel Brooks, what later became the Tower of Terror started out as "Castle Young Frankenstein" which would have featured a Bavarian village with winding streets to the castle with a drawbridge. The queue line would be towards the back of the "village" with a kind of indoor line that had the feel of Pirates of the Caribbean in the Magic Kingdom. The idea later changed to "Mel Brooks' Hollywood Horror Hotel".
Around the time the Disney-MGM Studios was opened, Bob Weiss wanted to do a 1930's art-deco high-end 4 or 5 diamond hotel at the front of the Disney-MGM Studios. Because murder mysteries were all the rage in the 1930's, McNair mentioned to Bob Weiss an idea for the hotel to build in little clues that could be found all around the grounds of the hotel and would be a mini scavenger hunt and had ideas like all the hotel staff would know little key phrases that would lead guests onto other clues and you could solve it and the end you could get a certificate that the guest had solved the mystery of the hotel. This idea lead to another idea of the Hollywood Hotel having one end being covered in ivy and had broken windows and was falling apart and when you went into the hotel, if you went down the hall to that section, it would say "closed" or "condemned" and that part would be the Hollywood Horror Hotel so that it was two buildings but it would look like it was one large building.
Some other ideas were tossed around, including an idea to incorporate Mel Brooks into the Studios was to do a "comedy Haunted Mansion" that would feature Castle Young Frankenstein on the same grounds in case the elevator based attraction was scrapped. Mel Brooks eventually left the project at the time Disney Imagineers had some firm ideas and 
brought in an idea to have a moving elevator off it's track and moving down hallways and crash out one side of the building and they had architects and engineers brought onto the project and Mel lost interest partially because Disney wasn't building on Mel's original idea anymore. Once Mel Brooks had left, Imagineers started to figure out what the new attraction would look like and budgets and so forth and played with it some more and with Mel leaving, Imagineers started leaning toward that Spanish-Renaissance/Riverside Mission Inn "look" because of the great architecture and because it would fit in with the eclectic store fronts on Sunset and Hollywood Boulevards. Disney felt the attraction needed a movie reference and eventually settled on the Twilight Zone theme after calling around and see what movie rights were available. The Twilight Zone theme was a fairly easy overlay for the Disney Imagineers and went through all the Twilight Zone episodes to pick elements from the series although the theme did not change the 
attraction much. The one element that was lost in all of this was the comedy aspect to it that Mel Brooks had wanted but with Mel gone, Imagineers focused on the eeriness and thrill of the attraction with a Twilight Zone theme to it.
One of the initial ideas for the Tower of Terror attraction was to have a bench near the queue with a man sitting down, hunched over with a newspaper in his lap and cobwebs between him and the newspaper and he would slightly move his head to the left and right and when he would sense a guest starring at him, he would "come to life", revealing he is a live actor and start conversing with the guest. There was another idea of having audio-animatronics including a elevator repair man sleeping in the boiler room. But this and other ideas were eventually scrapped because they slowed the line down too much, also because the line barely was in the lobby where the theme.
Now that they have their idea, the Imagineers had to think of a story to fit into theme of the park. After much consideration, the Imagineers finally found one 
that seemed to work, the plot revolved around a studio wrap party in a modern day hotel, where the owner started to kill the guests. Management knocked this back straight away due to the murder storyline. Next up was a narrated by Vincent Price, with a part walk through, part ride about a group of movie stars staying in a hotel and who disappeared during a storm. As the walk through progressed, clues would tell the story bit by bit, until you enter an elevator and what happened becomes all to obvious - and its too late to get out. Eisner liked this story, but wanted the guests to be more involved. And so the Twilight Zone theme was worked in, with guests starring in their own `lost` episode. The ride through portion of the attraction became the queue area - hence its great theming and little clues all around - and the boiler room holding area. Twilight Zone themes were worked into the attraction, but it became apparent the original freefall ride vehicle would have to do a lot more. Disney initially involved Otis elevators, who 
balked at the idea of a freefall car - they had spent 140 years making elevators feel like they were stationary.
The Fifth Dimension idea played a strong part of the story from the beginning - and it became the ideal transition from Ride shaft to drop shaft (early plans called for the 5th Dimension floor to be in the basement, having descended from the corridor scene, and then to rise to the top of the building and move into the drop shaft without a show scene.) To maximize capacity without duplicating everything it was decided to have 4 ride shafts but only 2 drop shafts. This, and the 5th Dimension transition, called for a new type of ride vehicle. Imagineering had to have an elevator car, but one that could also move horizontally. Disney had to think of their own ride system and vehicle, the Autonomous Guided Vehicle. A self controlling self contained ride vehicle, that could move without track. Although the 5th Dimension floor has guideways for traction, the vehicle itself runs on its own wheels along the floor. Such a vehicle needed to have onboard power, with fast charging. Inductive power coupling was designed for EPCOT Centers Universe of Energy travelling theatre cars by Inductran Corp, and can recharge onboard batteries without a physical connection. This technology was refined for Tower of Terror, and also used later in Tokyo Disneyland`s Poohs Honey Hunt. Onboard computers follow a pre programmed ride path, and `talk` to the Ride Control System via a wireless frequency. A secondary tracking system follows a wire embedded in the floor to keep track of the AGV`s location, and these can easily trigger a ride shutdown if a carelessly discarded park map comes between it and the underside of the AGV.
