The defense unit was the largest unit in S-21. The guards in this unit were mostly teenagers. Many guards found the unit's strict rules hard to obey. Guards were not allowed to talk to prisoners, to learn their names, or to beat them. They were also forbidden to observe or eavesdrop on interrogations, and they were expected to obey 30 regulations, which barred them from such things as taking naps, sitting down or leaning against a wall while on duty. They had to walk, guard, and examine everything carefully. Guards who made serious mistakes were arrested, interrogated, jailed and put to death. Most of the people employed at S-21 were terrified of making mistakes and feared being tortured and killed.