Sentence:  The original family who had begun to build a palace to rival 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 magic of a <WSD>promise</WSD> rather than the overpowering splendour of a great architectural achievement. 
Meaning of the word marked with <WSD> tags:  reason to expect improvement or success; potential