Here are the powers of the Mark of Sentinel.
Least
Mage armor, protection from arrows, shield of faith or shield other once a day, and increased skill in sensing deception and motive.[1]
Lesser
Protection from energy or lesser globe of invulnerability once a day.[1]
Greater
Globe of invulnerability once a day[1]
Siberys
Mind blank once a day[2]
Okay, don't know how Mind Blank is protective, but I thought of taking inspiration from the Armsmaster baselists.