Got a defender armored sentinel.  Was not impressed.  

Here's my feedback, bug portion underlined:

1- The Sentinel does not buff the caster.  You will see +10 defense over the caster's head if he's in range of the casting effect, but as soon as the first other unit acts (e.g. enemy) the effect is removed -10.  Probably a bug, causing the caster to lose it at the end of his current turn, rather than beginning of his next.

2- For others, the effect only gives +10 defense till their next action.  They could be the next to act, and immediately lose the effect.  +10 armor is such a small bonus, that to have it for only say half a turn is not worth taking the action.  Against a 10 point attack, 10 armor will only block 5 hp....  Against a 90 point attack 10 armor blocks about 9 hp.   If you already have a descent amount of armor, it blocks less because armor has diminishing returns.

3- The sentinel has two abilities, shield bash and arching shield bash.   Unfortunately shield bash tends to get weaker units killed, because bashing an opponent into another tile removes it from your units zone of control, and the AI is kind of dumb, and doesn't try to move its units when they are in melee range of a tank or summons - but bashing them away gives the AI a chance to play smart.

Sentinel is at the end of a defender tree branch, equivalent to grave elemental and Devlin pyre of men summons for mages.  That in mind, here are the changes I would make:

1. Sentinel will provide 2 armor bonus per Sentinel level to adjacent allies on an ongoing basis.  Thus, a lvl 5 sentinel provides 10 points of armor. Since sentinel level figures into the armor bonus, it synergies with the summoner trait and improved sentinel perk.

2. Sentinels level is further improved by 1 per earth shard.

3. Sentinel get's the guard ability and perhaps other cool unlocks if he's summoned at higher lvls.