12. 0-0-0
While castling is normally seen as a defensive move, White's king was in no danger here. Instead, Morphy used castling as an offensive weapon, as he now has a rook placed on the open d-file, placing even more pressure on Black's d7 knight.
It's worth noting that this move is stronger than playing Rd1. While the moves initially look similar, castling has the advantage of connecting White's rooks. If White had played Rd1, his king would still prevent the h1 rook from moving to the d-file. This small difference actually has a significant impact on White's attack.
12. ... Rd8
Black tries his best to hold the position, protecting his vulnerable knight.
13. Rxd7
White captures the knight. Weaker players often take with the least valuable piece they have without taking the position into consideration, but in this case, capturing with the rook is a far better move than Bxd7+. By leaving the bishop on the board, White will continue to apply pins and pressure the Black king after Black recaptures.
13 ... Rxd7
Black's only reasonable move is to take the rook and hope his defense holds up.
14. Rd1
This move shows why 12. 0-0-0 was much better than 12. Rd1. Now White can continue to bear down on Black's position and tie up his pieces.
A comparison of the two armies is telling. Every White piece is in an attacking position, while Black's pieces are pinned, blocked, or otherwise immobile.
14. ... Qe6
Black attempts to free his pieces somewhat, and brings us to our next diagram.
