sentences:  U-boats could also depart from Bruges at night, cutting a day off the journey to the Western Approaches, more easily avoiding the North Sea Mine Barrage and allowing U-boat captains to gain familiarity with the net and mine defences of the English Channel, through which they had to pass to reach the main battlegrounds of the Atlantic.In 1915–1916, the German navy had developed Bruges from a small Flanders port into a major naval centre with large concrete bunkers to shelter U-boats, extensive barracks and training facilities for U-boat crews, and similar facilities for other classes of raiding warship.Bruges was therefore a vital asset in the German navy's increasingly desperate struggle to prevent Britain from receiving food and matériel from the rest of the world.