Dragonpit

"When Targaryen fought Targaryen in the civil war called the Dance of the Dragons, an angry mob stormed the Dragonpit, that huge now-ruined vault where Targaryens stabled their beasts. Thousands died, but through sheer numbers and madness, five of the Targaryen dragons lay dead by the morning."

- Pycelle

The Dragonpit is a huge vault located in King's Landing. House Targaryen used it to stable their dragons.

History
During the Dance of the Dragons, a city-wide riot broke out, and the mob focused their anger on the dragons which were the source and symbol of power for the Targaryen kings. The mob - a vast wave numbering in the thousands - converged on the Dragonpit atop Rhaenys's Hill, the giant domed stable where the dragons were kept when not in use.

Many thousands of rioters died that night, but through their sheer numbers they managed to kill five dragons. The Dragonpit itself was left in ruins when the dragon Dreamfyre crashed into the ceiling in an attempt to escape, bringing down the entire structure, killing itself and many hundreds of rioters.

In the books
In the A Song of Ice and Fire novels, the Dragonpit is a huge, cavernous building that sits atop Rhaenys's hill in King's Landing. Prior to the Dragonpit, a great Sept was built on Rhaeny's Hill known as the Sept of Remembrance which used to be the main Sept in King's Landing. During the Faith Militant uprising, Maegor the Cruel mounted on Balerion the Black Dread incinerated the Great Sept with dragonflame and later decreed that a large domed structure would be built on the hill for Targaryens to house its royal dragons.

Thirty knights could ride abreast into its entrance. No pit dragon ever reached the size of the dragons who were raised before the construction of the building. When living dragons still nested beneath the dome light would shine through the windows at night.

King Aegon II's coronation took place in the Dragonpit. Later during the Targaryen civil war known as the Dance of the Dragons after King's Landing was captured by Queen Rhaenyra, the dragonpit was destroyed during the Storming of the Dragonpit. When tens of thousands of crazed and starving smallfolk led by the deranged prophet known as the Shepherd stormed the dragonpit to kill the dragons within. Five Targaryen dragons (Shrykos, Morghul, Tyraxes, Dreamfyre and Syrax) and thousands of smallfolk were killed. The dragonpit was reduced to flaming ruins.

During the Great Spring Sickness so many people died so quickly that there was no time to bury the bodies. Instead they were piled up in the Dragonpit and when the corpses were 10 feet deep the Hand of the King, Lord Brynden Rivers, ordered the pyromancers to burn them. The light of the fires shone through the windows and by night citizens could see the dark green glow of wildfire all through King's Landing.

Shortly after the outbreak of the War of the Five Kings, some whores use the Dragonpit as a place to entertain their customers and one of them, along with their patron, falls through the floor. There they find a hidden stash of wildfire, placed by Lord Rossart during Robert's Rebellion.