Preliminary assessment of the opening: White has driven a wedge into Black's camp which restricts Black's movement. It has not, however, hindered Black's development. The fact that Black has had time to fianchetto both bishops indicates that White has not been energetic enough. Black's pieces are positioned harmoniously to contest the center, whereas White's pieces, although enjoying more space, are not so well co-ordinated and lack a clear target. Whites' central pawn wedge, f4 and e5, is static, and White cannot occupy the apparent gaps in Black's camp, f6 and d6. Black gained compensatory ground on the queen side. White must inject some dynamic into his static center if he is to get anything going; hence, 13. d4.
13... cxd4 14. Nd1f2
If White retakes the d4 pawn immediately, then 14. ... Qc5 pinning the knight on d4 and threatening the undefended c2 pawn. White must therefore make several precautionary moves before retaking on d4. This gives Black several tempi to lay his plans.
Black now begins a new phase of the game: Black prepares to liquidate the center on his own terms. The rooks will therefore be brought to bear on the central files. White cannot retake on d4 immediately, as explained above; first, he must cover the c2 pawn.
Re-inforcing the c2 pawn but weakening the a3 pawn (as will be seen). The alternative was Bd3.
15... O-O 16. Rf1d1
White still cannot play 16. Nxd4 on account of ... NxNd4; 17. BxNd4, Qxa3. Thus he must recruit yet a third piece so as to retake with the rook on d4, leaving the bishop on b2 to hold the a3 pawn. This hands Black the opportunity he has been waiting for: to liquidate the central pawn wedge.
I probably thought longer and harder about this move than any other. My long-range plan is to convert the e-pawn to a passer. This rook move prepares to support its advance.