One of the most effective schemes for guarding a city from enemy bombers was that employed at Moscow, where concentric rings of antiaircraft batteries and searchlights filtered out the attackers before they could reach the heart of the city. The guns were carefully concealed, and ten or twelve of these circles of fire surrounded the capital, the outer ring more than 40 miles distant from the center of the metropolis. Each ring would drive off or shoot down part of the attacking force, and of perhaps 200 planes participating in a raid only a few would pierce the defenses. The multiple barrage of antiaircraft fire, barrage balloons and fighter planes combined to disperse or spoil the effective aim of the invading bombers.