In the campaign of 1645, Turenne was attacked with his army before Philipsburg by a very superior force. There was no bridge here over the Rhine, but he took advantage of the ground between the river and the place to establish his camp. This should serve as a lesson to engineer officers, not merely in the construction of fortresses, but of tÊtes de pont. A space should always be left between the fortress and the river, where an army may form and rally without being obliged to throw itself into the place, and thereby compromise its security. An army retiring upon Mayence before a pursuing enemy, is necessarily compromised; for this reason, because it requires more than a day to pass the bridge, and because the lines of Cassel are too confined to admit an army to remain there without being blocked up. Two hundred toises should have been left between that place and the Rhine. It is essential that all tÊtes de pont before great rivers should be constructed upon this principle, otherwise they will prove a very inefficient assistance to protect the passage of a retreating army. TÊtes de pont, as laid down in our schools, are of use only for small rivers, the passage of which is comparatively short.