Smalling said England's team ethic was also key in achieving the 1-1 draw against the balance of play.
"That togetherness has been here for the whole campaign. We never gave up and we really got stronger as the game went on. We felt we had the legs when it came to the last 20 minutes and it showed."
The centre-back wasn't all smiles, however, after Ander Herrera scored for Spain from a flicked-on corner. Smalling said: "It's disappointing to concede from a set piece. We'll have a look at that and make sure it doesn't happen again."