He has always thought of Kate Beckett as a warrior. Battlements full and prepared, ready to take on any enemy, ready to buck of any foreign invader and this -- simple, sweet surrender -- Castle barely has the means or will to explain it, her mouth parting beneath his, tasting of a brine as old as time itself. He's aware of the press of nails on the back of his neck and that's exactly his impetus to move forward, hands along her spine, his mouth responding in kind.