From A Wiki of Ice and Fire
The Dragonpit is a huge, cavernous building that sits atop Rhaenys's hill in King's Landing. The Targaryens would keep their dragons within the building. Its huge dome has collapsed within and its bronze doors have been sealed for more than a century. It is a ruin, blackened by fire. 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.
A Clash of Kings
Some whores were using the Dragonpit as a place to entertain their customers and one of them, along with their patron, fell through the floor. There they found a hidden stash of wildfire, placed by Lord Rossart during Robert's Rebellion.