The Morphy defense (3...a6) is the most common reply to the Ruy Lopez... it is "asking the question" to the Bishop

I opted for the exchange here, damaging Black's pawn structure but losing the Bishop pair (Ba4 is also common, heading for the better diagonal)

? Castles is the optimal move here, a fianchetto queenside is just stepping sideways

Undermining White's d.s. Bishop's diagonal and protecting e5

Defending f2 and preparing Queenside castle

Stopping the scary looking Nf4

This is playable due to Qc4+

? 15.Nxd4 exd4 16.f4 gives White some tempo and space on attacking Black's Kingside

? White missed this, instead opting for the equal trade

? An unsound knee-jerk reaction move, looking to sac the exchange for a potential mating sequence on the King i.e. 18...Bxd1 19.Nf7+ Kg8 20. Nh6++ and we have the philidor's mate (18.h4-h5 instead was better)