The experience of the Battle of the Somme led the German command to conclude that the policy of rigidly defending a trench position line was no longer effective against the firepower that the Entente armies had accumulated. Ludendorff published a new defensive doctrine in December 1916, in which deeper defences were to be built, within which the garrison would have room to manoeuvre, rather than rigidly holding successive lines of trenches. Along Vimy Ridge, the German forces spent two years constructing fortifications designed for rigid defence. Little reconstruction based upon the new defence-in-depth doctrine had been accomplished by April 1917 because the terrain made it impractical.