I meant to comment on this before but forgot. Some of it was the Mad King's left behind but Cersei had more made in season 2. I think the dragon attacking was setting off what was left behind.
She had a bunch made for sure.
Can’t really remember but maybe that was all for the battle where they blew up the ships in the bay and then the building full of Tyrells and priests.
I think everything under the city was Aerys’.
Regardless, I felt the slaughter by Danarys and her soldiers was pretty crazy.