Clay needed to design a version of the Dragonpit that would accurately represent what the structure might have looked like at the height of Targaryen power. Working from what had been previously established, he developed a formidable Brutalist structure with a domed roof that evoked ancient sites such as the Parthenon in Athens and the Colosseum in Rome. “It’s a colossal thing, hewn out of natural rock with recognizable columns and structure carved into the rock,” Clay says.

In terms of physical construction, however, all that was required to realize Clay’s design were several columns and corridors, a large platform, and a set of ornate, 2-foot-thick golden doors standing nearly 15 feet tall—the rest of the impressive space was created using visual effects based on Clay’s renderings. “The dragons enter the vaults through these tunnels that are meant to be something like 50 feet tall,” Clay says. “That architecture was based on Valyrian influences. It is the scale of the gods rather than the scale of humans.”

- Game of Thrones: House of the Dragon: Inside the Creation of a Targaryen Dynasty

 Across the city on the hill of Rhaenys stood the blackened walls of the Dragonpit, its huge dome collapsing into ruin, its bronze doors closed now for a century. The Street of the Sisters ran between them, straight as an arrow. The city walls rose in the distance, high and strong. 

- Book 1: A Game of Thrones

 From the high battlements of the gatehouse, the whole world spread out below them. Sansa could see the Great Sept of Baelor on Visenya’s hill, where her father had died. At the other end of the Street of the Sisters stood the fire-blackened ruins of the Dragonpit. 

- Book 1: A Game of Thrones

 “No, no,” Hallyne squeaked, “the sums are accurate, I swear. We have been, hmmm, most fortunate, my lord Hand. Another cache of Lord Rossart’s was found, more than three hundred jars. Under the Dragonpit! Some whores have been using the ruins to entertain their patrons, and one of them fell through a patch of rotted floor into a cellar. When he felt the jars, he mistook them for wine. He was so drunk he broke the seal and drank some.” 

“There was a prince who tried that once,” said Tyrion dryly. “I haven’t seen any dragons rising over the city, so it would seem it didn’t work this time either.” The Dragonpit atop the hill of Rhaenys had been abandoned for a century and a half. He supposed it was as good a place as any to store wildfire, and better than most, but it would have been nice if the late Lord Rossart had told someone. “Three hundred jars, you say? That still does not account for these totals. You are several thousand jars ahead of the best estimate you gave me when last we met.” 

- Book 2: A Clash of Kings

Hardly had the last stone been set on the Red Keep when Maegor commanded that the ruins of the Sept of Remembrance be cleared from the top of Rhaenys’s Hill, and with them the bones and ashes of the Warrior’s Sons who had perished there. In their place, he decreed, a great stone “stable for dragons” would be erected, a lair worthy of Balerion, Vhagar, and their get. Thus commenced the building of the Dragonpit. Perhaps unsurprisingly, it proved difficult to find builders, stonemasons, and laborers to work on the project. So many men ran off that the king was finally forced to use prisoners from the city’s dungeons as his workforce, under the supervision of builders brought in from Myr and Volantis.

- Fire and Blood "The Sons of the Dragon"

The wedding of the King’s Hand and the Queen Regent was to be as splendid as that of the widowed Queen Rhaena had been modest. The High Septon himself would perform the marriage rites, on the seventh day of the seventh moon of the new year. The site would be the half-completed Dragonpit, still open to the sky, whose rising tiers of stone benches would allow for tens of thousands to observe the nuptials. The celebrations would include a great tourney, seven days of feasts and frolics, and even a mock sea battle to be fought in the waters of Blackwater Bay.

- Fire and Blood "The Year of the Three Brides: 49 AC"

When the day of the wedding finally arrived, more than forty thousand smallfolk ascended the Hill of Rhaenys to the Dragonpit to bear witness to the union of the Queen Regent and the Hand.

- Fire and Blood "The Year of the Three Brides: 49 AC"

Yet for all the splendor of the bride and groom, it was the arrival of Alyssa’s children that set King’s Landing to talking for years to come. King Jaehaerys and Princess Alysanne were the last to appear, descending from a bright sky on their dragons, Vermithor and Silverwing (the Dragonpit still lacked the great dome that would be its crowning glory, it must be recalled), their great leathern wings stirring up clouds of sand as they came down side by side, to the awe and terror of the gathered multitudes.

- Fire and Blood "The Year of the Three Brides: 49 AC"

The young prince’s coronation and his mother’s Golden Wedding had both been splendid affairs that had done much to win him the love of lords and smallfolk alike, but all that had come at a cost. An even larger expense loomed ahead; Lord Rogar was determined to complete work on the Dragonpit before handing the city and the kingdom over to Jaehaerys, but the funds were lacking.

...

The new levies did, however, serve to make Lord Celtigar loathed throughout the city. Lord Rogar and Queen Alyssa received their share of opprobrium as well. Another casualty was the Dragonpit; the Crown no longer had the funds to pay the builders, and all work on the great dome ceased.

- Fire and Blood "A Surfeit of Rulers"

The securing of the loans had one immediate effect; work on the Dragonpit was able to resume, and once again a small army of builders and stonemasons swarmed over the Hill of Rhaenys.

- Fire and Blood "A Surfeit of Rulers"

Building was the king’s other concern that year. Work continued on the Dragonpit, and Jaehaerys oft visited the site to see the progress with his own eyes.

- Fire and Blood "Birth, Death, and Betrayal Under King Jaehaerys I"

As the Year of the Stranger neared its end, work on the Dragonpit was all but complete. The great dome in place at last, the massive bronze gates hung, the cavernous edifice dominated the city from the crown of Rhaenys’s Hill, second only to the Red Keep upon Aegon’s High Hill.

- Fire and Blood "Birth, Death, and Betrayal Under King Jaehaerys I"

The tourney that Lord Redwyne had proposed to celebrate the completion of the Dragonpit was finally held at midyear.

...

No dragons had been settled in the Dragonpit as yet, so that colossal edifice was chosen for the site of the tourney’s grand melee, a clash of arms such as King’s Landing had never seen before.

- Fire and Blood "Jaehaerys and Alysanne: Their Triumphs and Tragedies"

Not long thereafter Balerion became the first of the Targaryen dragons to be housed in the Dragonpit. Its long brick-lined tunnels, sunk deep into the hillside, had been fashioned after the manner of caves, and were five times as large as the lairs on Dragonstone. Three younger dragons soon joined the Black Dread under the Hill of Rhaenys, whilst Vermithor and Silverwing remained at the Red Keep, close to their riders. To ascertain there would be no repetition of Princess Aerea’s escape on Balerion, the king decreed that all the dragons should be guarded night and day, regardless of where they laired. A new order of guards was created for this purpose: the Dragonkeepers, seventy-seven strong and clad in suits of gleaming black armor, their helms crested by a row of dragon scales that continued, diminishing, down their backs.

- Fire and Blood "Jaehaerys and Alysanne: Their Triumphs and Tragedies"

Prince Aemon emerged victorious, in part by dint of hammering his brother into submission. Later he distinguished himself in the lists as well, and was awarded his knight’s spurs in recognition of his skills. He was seventeen years of age. With knighthood now achieved, the prince wasted no time becoming a dragonrider as well, ascending into the sky for the first time not long after his return to King’s Landing. His mount was blood-red Caraxes, fiercest of all the young dragons in the Dragonpit. The Dragonkeepers, who knew the denizens of the pit better than anyone, called him the Blood Wyrm.

- Fire and Blood "The Long Reign - Jaehaerys and Alysanne: Policy, Progeny, and Pain"

Nor was her brave prince the only mount the princess was to claim that year. Like her brothers before her, Alyssa Targaryen meant to be a dragonrider, and sooner rather than later. Aemon had flown at seventeen, Baelon at sixteen. Alyssa meant to do it at fifteen. According to the tales set down by the Dragonkeepers, it was all that they could do to persuade her not to claim Balerion. “He is old and slow, Princess,” they had to tell her. “Surely you want a swifter mount.” In the end they prevailed, and Princess Alyssa ascended into the sky upon Meleys, a splendid scarlet she-dragon, never before ridden. “Red maidens, the two of us,” the princess boasted, laughing, “but now we’ve both been mounted.”

The princess was seldom long away from the Dragonpit after that day. Flying was the second sweetest thing in the world, she would oft say, and the very sweetest thing could not be mentioned in the company of ladies. The Dragonkeepers had not been wrong; Meleys was as swift a dragon as Westeros had ever seen, easily outpacing Caraxes and Vhagar when she and her brothers flew together.

- Fire and Blood "The Long Reign - Jaehaerys and Alysanne: Policy, Progeny, and Pain"

That very night, Princess Saera sealed her fate. Instead of remaining in her rooms as she had been instructed, she slipped away whilst visiting the privy, donned a washerwoman’s robes, stole a horse from the stables, and escaped the castle. She got halfway across the city, to the Hill of Rhaenys, but as she tried to enter the Dragonpit, she was found and taken by the Dragonkeepers and returned to the Red Keep.

Alysanne wept when she heard, for she knew her cause was hopeless. Jaehaerys was hard as stone. “Saera with a dragon,” was all he had to say. “Would she have taken Balerion as well, I wonder?”

- Fire and Blood "The Long Reign - Jaehaerys and Alysanne: Policy, Progeny, and Pain"

Even as a little girl, Viserra had been the most beautiful of the queen’s daughters. Great lords, famous knights, and callow boys had danced attendance on her all her life, feeding her vanity until it became a raging fire. Her great delight in life was playing one boy off against the other, goading them into foolish quests and contests. To win her favor for a joust, she made admiring squires swim the Blackwater Rush, climb the Tower of the Hand, or set free all the ravens in the rookery. Once she took six boys to the Dragonpit and told them she would give her maidenhead to whoever put his head in a dragon’s mouth, but the gods were good that day and the Dragonkeepers put an end to that.

- Fire and Blood "The Long Reign - Jaehaerys and Alysanne: Policy, Progeny, and Pain"

In 93 AC, Prince Baelon’s sixteen-year-old son, Viserys, entered the Dragonpit and claimed Balerion. The old dragon had stopped growing at last, but he was sluggish and heavy and hard to rouse, and he struggled when Viserys urged him up into the air. The young prince flew thrice around the city before landing again. He had intended to fly to Dragonstone, he told his father afterward, but he did not think the Black Dread had the strength for it.

Less than a year later, Balerion was gone. “The last living creature in all the world who saw Valyria in its glory,” wrote Septon Barth.

- Fire and Blood "The Long Reign - Jaehaerys and Alysanne: Policy, Progeny, and Pain"

Screenshot Timeline of the "Lesser" Dragonpit in Game of Thrones Season 7