Fortress?

During a blitz game this afternoon, I constructed what I hoped was a fortress. The time control was 5 minutes plus a three second delay. I was playing on the increment with 17 seconds left for a long time. My opponent was burning time trying to find a winning idea. When he was down to 3 seconds and I still had 7, I suggested a draw. He wasn’t convinced it was a fortress, but accepted the draw.

White to move

How can White break down Black’s defense?