Because of its strategic significance, the Confederacy relocated its capital to Richmond. Richmond was at the end of a long supply line and as the highly symbolic capital of the Confederacy became the main target of round after round of invasion attempts. A major center of iron production during the civil war was located in Richmond at Tredegar Iron Works, which produced most of the artillery for the war. The city was the site of numerous army hospitals. Libby Prison for captured Union officers gained an infamous reputation for the overcrowded and harsh conditions, with a high death rate. Richmond's main defenses were trenches built surrounding it down towards the nearby city of Petersburg. Saltville was a primary source of Confederate salt (critical for food preservation) during the war, leading to the two Battles of Saltville.