Getting There
Airlines servicing Cluj airport include Wizz, Blue Air, Lufthansa, Austrian Airlines and Turkish Airlines. From there treat yourself and get a taxi direct to site. It'll cost under $20 and that, friends, is what they call a no-brainer.
Lodging
If you can't bear the thought of sleeping in a tent, it's a 30-minute taxi back to Cluj from Bánffy Castle. The Hampton by Hilton is affordable. For more contemporary and less cash, check the Hotel Paradis. For cheap as chips, a bed in an eight-person dorm in Transylvania Hostel will hardly set you back.
Leave Your Beers at Home
You can't take outside alcohol onsite with you, but beers are only just over a dollar each so this isn't much of a hardship. You can bring food, though.
You Won't be Allowed Inside the Castle
There's some selected parts used for stages and areas, but most of Bánffy Castle is cordoned off so don't expect to be able to run around it doing your scariest Dracula cackle.
Bring Sunscreen and Wellies
The Cluj weather in July is hot, so don't forget to pack protection. Last year it also rained several buckets, so best be prepared for all eventualities (or at least budget for buying some onsite).