How am I going to stop some big mean mother hubbard from tearing me a structurally superfluous new behind? The answer: use a gun. And if that don't work... use more gun.
Sentry Guns, also known as Turrets or Sentries are a building type built by the Engineer. Just like other Buildings (except the Jump Pad) they have 3 levels, and each level costs 200 metal to upgrade. The Gun itself is a device on a tripod constructed of metal and team colored-paint and locks on to enemies with perfect accuracy.
A level 1 Sentry Gun is an automatic single machine gun with an internal magazine with a team-colored top with a team-colored light to the left side of the barrel.
A level 2 Sentry Gun has 2 gatling guns on either side and is belt-fed ammunition. It appears to use an automatic cranking system to rotate the barrel, as indicated by the 2 electric cables visible.
A level 3 Sentry Gun is identical to level 2, except for a rocket launcher which comes out of the top of the Sentry Gun and fires 4 rockets at once.
The Sentry Gun is a powerful force for area denial. The Sentry Gun has perfect accuracy and an incredible amount of damage potential, as well as noticeable knockback. It locks on to the closest target within range and line of sight and opens fire until the threat is out of sight, out of range or neutralized. A well-placed Sentry Gun can singlehandedly defend a control point, an intelligence briefcase a Payload cart or anything else.
However, they take significant time to set up, have limited range, are not very mobile, and can be easily destroyed via an ÜberCharge push, a Spy's Sapper, the Demoman or any significant source of damage from range. Even the Scout's Pistol can destroy an unattended Sentry if he finds the right spot to shoot from.
The Sentry Gun costs 130 metal to build, by far the most expensive building for the Engineer to construct, and just like any other building (except the Jump Pad) it takes 200 metal to increase to the next level, totaling in 530 metal to fully construct and upgrade a Sentry. With each level the Sentry Gun becomes stronger, gaining additional guns, rockets and health. However, the range does not change with level. The Sentry Gun's health and ammo can be replenished by any allied Engineer hitting it with a Wrench. This allows a team with multiple Engineers to all support each other. Allied Engineers, however, cannot move another Engineer’s Sentry.
When a Sentry Gun is placed, it starts with 0 health but like all Buildings it gains health as it constructs. This continues until it reaches its maximum health. Any Sentry Gun routinely rotates 90 degrees around itself, however it will lock on any enemy entering its range, even if it was behind the Sentry Gun. However, the Gun turning takes some time, which can allow enemies to slip through. The Sentry Gun beeps as it turns, and the beeping increases with level upgrades. Experience players can use this to detect a Sentry Gun before seeing it and know its level before engaging.
A Sentry Gun makes beeping noises that alert other players to their presence. Each time it turns, it makes an audible beeping noise that increases based on level. The number of beeps corresponds to the Sentry’s level, a level 1 makes 1 beep, level 2 makes 2, and so on. When a Sentry Gun detects an enemy player in its range, it makes a distinctive noise and locks on the enemy before firing.
A Sentry constantly checks if an enemy player is within its If there are multiple range and line of sight. If there are multiple targets that meet these criteria it focuses on the closest target. But if another enemy gets closer it changes targets and shoots that one instead. Sentry Guns also only fire once they are in line with a Sentry, even if firing could damage the enemy. This can be utilized, for example, as a Scout to run in circles around a Sentry and not being fired on. Sentry Guns have a maximum turning speed which increases with level.
When a Sentry Gun is hauled and placed down again, it must go through its construction and upgrade animations before it can function, similarly to a teleporter. This makes a level 3 Sentry ill-suited for offensive situations where the fight is constantly moving, such as offense on Payload maps, as a Sentry Gun that is re-constructing itself is a very easy target. If the fight is constantly moving or the Engineer is on offense, an Engineer can consider simple using a level 1 Sentry that can be easily moved around as needed. If an Engineer is killed while hauling a Sentry (or any Building) it gets destroyed with a special kill icon.
The Minigun has a unique 15% damage penalty against level 2 Sentries and a 20% damage penalty against level 3 Sentries.
Sentries have various vulnerabilities players can exploit to negate its damage or destroy it. A weapon fired at a Sentry Gun does not suffer from damage fall-off, allowing an enemy player to destroy it even from long distances. A destroyed Sentry drops metal parts which can be collected by any player as ammo or metal.
Sentries will not fire on cloaked or disguised Spies. However if a Spy disguises while it is being targeted by a Sentry, it will continue firing unless the Spy breaks line of sight or escapes it’s range. The Sentry will also not fire if the enemy is hidden behind cover, even if they are in the Sentry Guns range. Sentries are also not able to fire if they are being sapped or if they are in the middle of being constructed/upgraded.
Engineers are also able to take self-damage from their own Sentry. Engineers looking to avoid this can stand behind their Sentry Gun and not in front of it.