This is a stand-alone novel set in what is roughly the US after the collapse of modern civilization and its replacement by cultures which frankly seem to me to be too much like pre-modern Europe/Eurasia to be probable. A nomadic culture like Huns or Mongols is loving in on a settled agricultural culture like pre-modern western Europe. Some youths from the culture under attack find that a ruined city (whose people live chiefly by selling scrap from the ruins) includes a vault in which a far-seeing man of the fallen civilization had left a large quantity of boh information and actual machines (in graduated stages) to assist in rebuilding civilization. This becomes involved in te war with the nomads, which the setters finally win. ( )

A very early novella reprinted - as were so many of Anderson's works - in the 1970s. Some of his standard themes are very much in evidence, including a Libertarian organization of the post-apocalyptic tribe, male heroism, and the value and beauty of man-made technology. ( )