Sentence:  The original family who had begun to build a palace to outrival Nonesuch had died out before they had put up little more than the gateway, so that the actual structure which had come down to posterity retained the secret <WSD>magic</WSD> of a promise rather than the overpowering splendour of a great architectural achievement. 
Meaning of the word marked with <WSD> tags:  Something producing successful and remarkable results, especially when not fully understood; an enchanting quality; exceptional skill. (from 17th c.)