sentry n.

1570s, "a soldier stationed as a guard;" 1580s, "watch or guard kept by a soldier stationed for that purpose;" from French sentinelle (16c.), from Italian sentinella "a sentinel." OED writes that "No convincing etymology of the It. word has been proposed," but perhaps it is (via a notion of "perceive, watch"), from sentire "to hear," from Latin sentire "feel, perceive by the senses" (see sense (n.)). As an adjective by 1650s.

early 14c., seintuarie, sentwary, etc., "consecrated place, building set apart for holy worship; holy or sacred object," from Anglo-French sentuarie, Old French saintuaire "sacred relic, holy thing; reliquary, sanctuary," from Late Latin sanctuarium "a sacred place, shrine" (especially the Hebrew Holy of Holies in the temple in Jerusalem; see sanctum), also "a private room;" in Medieval Latin also "a church, cemetery; right of asylum," from Latin sanctus "holy" (see saint (n.)). 

Since the time of Constantine and by medieval Church law, fugitives or debtors enjoyed immunity from arrest and ordinary operations of the law in certain churches, hence its use by mid-14. of churches or other holy places with a view to their inviolability. The transferred sense of "immunity from punishment by virtue of having taken refuge in a church or similar building" is by early 15c., also of the right to such. (Exceptions were made in England in cases of treason and sacrilege.)

The general (non-ecclesiastical) sense of "place of refuge or protection" is attested from 1560s; as "land set aside for wild plants or animals to breed and live" it is recorded by 1879 in reference to the American bison.