A sentinel is a person, such as a sentry, assigned to keep guard or watch. Sentinels must be vigilant and untiring, ready to sound the alarm and/or repel the enemy. Their failure to do so can have catastrophic results. They must be well-disciplined and of good judgement, able to make split-second decisions correctly. Thus, they are usually chosen from among the best, most trustworthy soldiers. Their duty is dangerous, boring, and tiring, but they do it gladly, out of a sense of duty and a love for that which they protect.