Because Percy has already gone to Hades' palace and it is the same as Zeus' but darker and he realises that its like that because Hades is always left out and according to the start of the first book: Percy Jackson and the lightning thief, Hades is only allowed to visit Mount Olympus twice a year.
Percy starts to feel some empathy for Hades. He thinks maybe Hades got a bad deal.
Because Percy has already gone to Hades' palace and it is the same as Zeus' but darker and he realises that its like that because Hades is always left out and according to the start of the first book: Percy Jackson and the lightning thief, Hades is only allowed to visit Mount Olympus twice a year.
It is often just referred to as The Palace of Hades.
Attached link is to vase art of Hades palace.
Percy wanted to barter with Hades because his mother was abducted by the Minotaur, that was sent by Hades to destroy Percy. Percy wanted to get his mother back, so he went to the underworld to barter with Hades. However, he was also sent to barter with Hades for the lightning bolt. Hades was accused of possessing the bolt, so Percy was sent to the underworld to get the bolt back.
Greek myth does not say what the palace of Hades is made of.
Hades sent the Minotaur to abduct Percy's mother.
Hades
The palace in the Underworld.
Cerberus.
Hades Crown.
Poseidon had a palace on Olympus, but Hades did not.