The most effective way to protect walls against cannonfire proved to be depth (increasing the width of the defences) and angles (ensuring that attackers could only fire on walls at an oblique angle, not square on). Initially, walls were lowered and backed, in front and behind, with earth. Towers were reformed into triangular bastions. This design matured into the "trace italienne". Star-shaped fortresses surrounding towns and even cities with outlying defences proved very difficult to capture, even for a well-equipped army. Fortresses built in this style throughout the 16th century did not become fully obsolete until the 19th century, and were still in use throughout World War I (though modified for 20th-century warfare). During World War II, "trace italienne" fortresses could still present a formidable challenge, for example, in the last days of World War II, during the Battle in Berlin, that saw some of the heaviest urban fighting of the war, the Soviets did not attempt to storm the Spandau Citadel (built between 1559 and 1594), but chose to invest it and negotiate its surrender.