I think it's Gardenia's and Cheryl's (I think that's Chansey girl's name) secret base of operations. I bet there's a secret button on the statue that opens a staircase in the floor, that leads to their evil lair. The ghosts are caused by projectors to scare trainers away from the mansion.
The human ghosts, that is. Obviously Cheryl and Gardenia have captured ghost pokemon from the Pokemon Tower and bred them inside the Old Chateau. Except for the one Rotom the brought. That way either the ghost pokemon would scare them, or once they found Rotom they'd think that the trainer had found the secret of the Old Chateau, and they'd leave.
Think about it, Cheryl and Gardenia are the only two people besides you that have been seen near the Old Chateau. And isn't it a bit suspicious that there are so many pairs of trainers throught the forest, obviously hired hands to keep feeble trainers from passing through the forest. Cheryl would guide any trainer with actual potential right past the Chateua, right to Eterna forest so they wouldn't notice. Instead they'd only see some trees scattered among a worn fence. Though once they got to Eterna City the average trainer would be too occupied battling Cheryl's accomplice, noneother than the gym leader Gardenia, and forget all about the mysterious fence.
