Actually, I live in London. So we could meet wherever you would like to talk.
Since it is a castle I'd need to know approximately the size of it. I could spell some sort of protection circle around the perimeter. If anyone tries to breach it using magic or a spell of any source I could make an amulet for you. The amulet would turn red if there was a threat or danger.
The spell would need to be renewed every year. Or at least the barrier would need to be checked to see if it had been tampered with in any way.