In the same book, page 24

for, it is necessary to remark, that the Bormida, though generally rapid and deep, was nevertheless fordable at several places. The enemy was particularly tenacious of its position near the bridge; but the principal point of action was a San Stefano. From this point the enemy could gain Voguera before we could, and thereby cut off our retreat.

In the original version, Marengo, ou Campagne d'Italie par l'armée de réserve commandée par le général Bonaparte, p. 57.

Car, il faut remarquer que la Bormida, quoique rapide et profonde, était, néanmoins, guéable en plusieurs endroits. Les ennemis inarquaient vers le pont un acharnement incroyable; mais le point principal de action était San-Stéfano. De cet endroit, ils pouvaient gagner Voghéra avant nous; et nous couper toute retraite.

It looks like a place near the river, and might have a better road to Voghera than Marengo, so as you have said in the email, Castelceriolo probably.