There’s a list of 18 castles: 9 of your own and 9 belonging to your opponent. They are paired off, more precisely 1-2-2...-2-1, until only two of your own and two of your opponent’s castles remain, which are then taken. You can pair off your own or your opponent’s castles in any order.
For example:
First: your Rampart
Second: your opponent’s Necropolis, your Dungeon
First: your opponent’s Castle, your Cove
And so on.

P.S.: The one with the Necropolis will be the luckiest.