Why you should use sentries

In the low leagues of Starcraft 2, a few units from each race are underused - not because the Units are bad, but seem to need a lot of attention to be effective. Units like the infestor, the raven, the ghost and most importantly, the sentry.

The incredible versatility of of the unit greatly makes up for its underwhelming damage - which is only 1 higher than that of a probe - and their health is comparable as well. However, they have their special abilities which rely on the presence of energy: force field, hallucinate and guardian shield. All of these unpowered are consistently useful throughout the different stages of the game.

Sentries are used effectively about as frequently as stim is in the low leagues. They're nearly never seen in bronze and gold league play, which is why including them will make a huge difference in a protoss player.

In the beginning of any game, including about 3 sentries in your initial gateway unit will make any pressure - 4 gate, ling/roach rush, marine rush - much easier to deal with and can allow them to expand without as much risk. With these three sentries, the player can consistently keep the rushing units at bay until they are prepared to confront them. When force fielding a normal ramp - such as that at the ramp to the natural on Ohana, shakuras or the ramps up to the main bases on all of the current maps - you have the option to either split the enemy units, keeping the bulk of them on a lower level and being able to pull back the lucky few you let up back from the ramp to prevent any ranged units on the low ground from causing damage.

 

Like this, but with fewer forcefields

In a rush scenario, you likely wouldn't want to spend energy on guardian shield unless you have either extra sentries. Guardian shield is most useful when you are in control of the fight. One thing players in the low leagues must realize is that guardian shield will not lessen melee damage, do not use guardian shield against zerglings or zealots.

Following the potential rush timings, scouting is crucial to success. When against Terran or Protoss players who are walled in, the standard entry with a probe will not work. Instead of sending an observer in, sentries can use some of the energy they will have pooled to create a hallucination - such as Phoenix- to see the opponent is doing. Prior to this, the player must research the upgrade at the cybernetics core. Flying a hallucinated Phoenix in will both provide invaluable scouting information and potentially cause the opponent to believe you to be teching towards air units. Currently in the heart of the swarm beta, the upgrade for hallucinate is no longer necessary to use he ability.

Because knowing what your enemy is doing helps sometimes!

The late game of pvp and pvt at this time tend to come down to large 200 supply ground armies fighting, micro and upgrades deciding the winner. Sentries will often elevate a definite in upgrades or army supply in these scenarios. In pvt, when against a large bio ball - marines, marauders, mediacs - force fields and guardian shields will minimalism th damage your army takes. In the lower leagues where macro may be lacking, force fields and guardian shield keeping units alive will be incredibly beneficial.

Using force fields against large armies encourages you to be both efficient with your force fields and to plan where they're going to go. Bringing the enemy units into a tighter choke, or somewhere you can prevent other units from getting past the force fields efficiently will allow you to essentially split up their army. The idea is simple. The execution, not incredibly. Executing these sorts of things require you to practice force field placement.

Depending on your unit composition, you will want to force field in a slightly different manner. If you're building Templars with storm, you may want to force field in a slightly concave line, so that units running forward to avoid the psi storms will take damage for slightly longer. If you're building colossus, forcefielding in a straight line will cause AoE damage from the colossus to be effective as units run to he side to micro back.

Because splash damage is useful

After forcefielding while fighting the now divided army, casting guardian shield will be very useful when dealing with ranged units. Of course, if your enemy he only zealots or zerglings, don't bother with guardian shield as it will be a waste of energy. Hallucinations will be useful now, as well if you have too many sentries, as you can make some fake archons or colossus to tank some damage and to continue benefiting your army and to keep more of your expensive units alive.

The sentry will allow you to be in control of the engagements and to allow you to decide when you would like to fight. It allows you to scout relatively inexpensively and exploit players who do not use detectors. It allows you to divide armies into nicer sizes and protect yourself from ranged attacks. If you're stuck in the lower leagues as a protoss player, and you're not building sentries, reevaluate your play style and try to include this unit.