At the start of the series, Sentinels are the elite troops of a number of armies. They have been seen in use by the Wasp Empire, varous ant City-States and the city guard of Collegium. Their most notable feature is their very heavy armour, known as Sentinel Plate. In many circumstances Sentinels are allowed to arm themselves as they see fit, a consequence of their elite status. Units are known for their tenacity and refusal to break.
With the introduction of the snapbow, however, sentinels start to go out of favour since even that armour is unable to turn aside the bolts from that weapon. By the time of Heirs of the Blade the Wasp armies have been decommissioning sentinel units in favour of more snapbowmen.