Concise Bible Dictionary:

1. mishmaath, from “obedience,” a body-guard (2 Sam. 23:23; 1 Chron. 11:25).
2. mishmar, a place where watch was kept (Neh. 4:22-23; Ezek. 38:7).
3. ruts, “to run,” runners, state couriers, who published edicts in the provinces, some of which would always be with the king (1 Kings 14:27-28; 2 Kings 10:25; 2 Kings 11:4-19; 2 Chron. 12:10-11).
4. tabbach, slaughterer, executioner, attached to the body-guard of the king (Gen. 37:36; Dan. 2:14). This Hebrew word occurs in every other passage where the word “guard” occurs.

From Manners and Customs of the Bible:

Matthew 27:36. Sitting down they watched him there.
This refers to the military guard, who were the actual executioners, and whose duty it was to watch the person crucified lest his friends should rescue him. In this instance the guard probably consisted of five men, four of them ordinary soldiers (a “quaternion”; see note on Acts 12:4, #830) and the fifth the centurion in command. Compare Matthew 27:54 with John 19:23.