The sentry has the ability to cast a huge forcefield. It's relatively cheap, so it's a pretty good investment at any stage of the game, but especially early-mid. If your team controls the high ground, this is the perfect unit for you, but without high ground control it's still quite useful, but nowhere near as useful as it has the potential to be.

      Very very good investment... only be really careful with him cause he's got about the same amount of health as a marauder.