 The Irben Strait, the main southern entrance to the Gulf of Riga, was heavily mined and defended by a number of coastal artillery batteries.The garrison on Ösel numbered nearly 14,000 men, though by 1917 it had been reduced to 60 to 70 percent strength.The operation began on 12 October, when "Moltke" and the four "König"-class ships covered the landing of ground troops by suppressing the shore batteries covering Tagga Bay.