The sentinel is a soldier armed and trained for defending the walls of castles and forts. His primary weapon is the bow, for thinning the numbers of besiegers as they approach the walls. Then when the enemy closes he resorts to sword and shield.
The common archer build for Skyrim will be some variation of a stealth archer. The stealth archer will be lightly armoured, relying on stealth for defence and sneak attacks to maximise damage. The sentinel however is an archer that instead relies on sturdy armour for defence and sword skills to supplement damage output. The sentinel is an armoured archer and capable swordsmen.
The sentinel is a warrior that uses his ranged power to force opponents to close with him. If the enemy survives the storm of arrows to close the distance the sentinel will not retreat or hide but switch to sword and shield to grind out a victory.
Against enemy archers the sentinel can trade arrows with the foe winning on attrition due to his superior armour. Or else, he can advance to melee range using armour and shield to tank out incoming fire.
The sentinel sees far and moves slowly but not stealthily. He chooses his ground carefully, all the better to force his enemies to come to him at their disadvantage. The sentinel chooses the high ground and holds the choke point.
The only perks not relevant to the sentinel’s tactics is the Ranger perk. This perk allows the character to move faster with bow drawn. All other archery perks will be of use to a sentinel.
The sentinel does not retreat before enemies that can weather his hail of arrows. He holds his ground and switches to the sword and shield to finish the job.
One handed perks will be generally useful. However the sentinel will grind out his opponents from a static position using the benefit of his armour and shield. Consequently the sentinel will have less use for the Critical Charge perk and the perks related to dual wielding.
The use of a shield is a key part of the sentinel’s swordstyle. Most block perks will be of use. However the sentinel’s defensive style would depreaciate the block perks that enhance an aggressive advance such as Block Runner and Shield Charge.
The defensive and static style of fighting of the sentinel suggests sacrificing mobility and stealth for better protection in armour. Heavy armour is to be preferred over light.
As with any warrior build, the importance of arms and armour does suggest investing in the skills relating to their maintenance and improvement.
The sentinel described above is a pure warrior build. Just as with any warrior build which invests in smithing skills it can be a good investment to also take up enchanting too.
A spellsword variant could be created by swapping out archery for the ranged attacks of the Destruction school of magic. The runes of this school also have value in the sentinel fighting style.
It is tempting with an archer build to opt for a companion that is more tilted towards close combat. This kind of companion can draw enemies away from your archer allowing for more shots before needing to relocate.
The sentinel however is able to handle himself when battle comes to melee range. Thus an archer companion may be better. Such a companion will supplement your own ranged attacks while not advancing too much off your chosen ground.
Feandal, Aela, Jenassa are all good archer companions. Valdimar deserves and honorable mention. Although not an archer, Valdimar will sling ice spikes which besides being a ranged attack also slows enemies’ movement.
The sentinel’s skillset significantly overlaps with the Perfected Dragonborn build that I have identified here.
The early Fus Ro Dah shout of Unrelenting Force is a particular use to the sentinel for creating space for more archery on opponents that close to melee.
If you like to dely or avoid dragonborn status then the sentinel can find good employment in the Companions or in either of the Civil War factions.
Otherwise the sentinel does rather better in battles above ground than under it. The sentinel can use his ranged power to draw bandits out of their camps for example. In dungeons the sentinel will tend to be forced into close quarters sooner.