The Engineer's Sentry Bot is the Engineer's primary badge. You can spawn a stationary Sentry Bot by holding the left arrow button for a few seconds. It fires bullets at enemies, fires rockets at Drones, and is able to spot disguised saboteurs and cloaked scouts.
A recent patch (1.21) increased its power, as it was proven to be too weak.
In multiplayer, the drones are also functioned as base camp defense turrets. The difference is that every time you destroyed the turrets, they will "resurrect" automatically.