Secrets of Effective Engineering
1. Rotating Buildings: By default, right-clicking while placing a building will rotate it 90° at a time. It is possible to place buildings (particularly sentries and teleporters) in corners and adjacent to walls while facing away from them using this method.
2. Building a Teleporter Exit: It is possible to place a teleporter exit in addition to the entrance. By default, this is done by pressing '4' and then '4' again. Doing so will link the entrance to the exit and allow your teammates to travel quickly between the two. This must be done for teleporters to maintain proper functionality.
3. Helping Your Fellow Engineers: It is possible to assist your Engineer teammates by whacking their buildings with your melee weapon to upgrade or repair them. In particular, even if you are not an Engineer, becoming one temporarily to upgrade the teleporter entrance of another is very helpful. Doing this may earn you an achievement or two in-game as well!
4. Firing Your Pistol: Swapping to your pistol (done through either pressing '2' or scrolling the mouse wheel by default) can allow you to target mid-range enemies or strike swiftly if your Shotgun runs out of ammunition, provided you are not instead using the Wrangler or the Short-Circuit. Unlike the Scout, the Engineer has plenty of reserve ammunition (200!) for his pistol and can safely fire it almost constantly without worrying about running out.
5. Assessing the Validity of Teammates: Not all those who appear to be your teammates are, in fact, teammates. Enemy spies disguised as members of your team will often target Engineers almost exclusively. If you see a member of your own team purposefully avoiding collision with other members of your team and/or walking straight toward your gear, it is usually wise to attack that person and assess the structural stability of his/her skeleton versus your weaponry. True members of your own team will not be harmed by the special science inherent in your bullets and melee attacks.
6. The Better the Sentry, the More It Beeps: Normal sentries lain by an Engineer can be upgraded to level-2 and later level-3 versions by bludgeoning them with a melee weapon. These sentries gain firepower (and later missiles), but beep louder and more frequently, alerting shrewd enemies to their presence beforehand. Level-1 sentries beep only once each time their 'patrol' AI reverses direction, while level-2 and level-3 sentries beep twice and thrice respectively.
7. Relocating Structures: Unless you are wielding an unlockable wrench called 'The Eureka Effect', it is possible to move any of your buildings, fully intact, to another location. Simply stand next to the building you wish to move and right-click to pick it up, then walk around and place it as you would any other building. It is important to note, however, that you are slowed down while carrying a building- and completely vulnerable to enemy attack. Furthermore, if you are killed while carrying one, the building is destroyed as well!
8. Teleportation Glow: Upon using a teleporter, you (and any of your teammates) will gain a trail of glowing particles around your feet for a short time. Because of this, it is often very difficult to hide teleporters; enemies can be alerted to their presence simply through observation. Make sure you defend them well!
9. Telefragging: Under most normal circumstances, it is not possible for a member of your team and a member of the enemy team to occupy the same space at the same time. If an enemy is on your teleporter exit when you use it, that enemy will fall over dead immediately and you will be credited with a "Telefrag" kill. More importantly, enemy spies are capable of traveling through your teleporters as well! If you are standing on the other end when one comes through (sometimes the Spy will trickily sap one end of the teleporter first), you will die instead. Make sure to avoid standing directly on top of your own teleporter exit at all times!
10. Scavenging Metal: While at first glance it is rather easy to find the wooden ammo crates scattered throughout most stages for extra metal, you can also pick up fallen players' weaponry- both from your own team and the enemy team- to restore metal and ammunition. In addition, whenever one of your buildings is destroyed, metallic pieces of it scatter around on the ground. Picking these up restores some (but not all) of the metal used to construct the building.
11. Standing on Buildings: Although anyone can stand on top of one of your teleporters, only you and members of the enemy team can stand on top of one of your own sentries or dispensers. If you ever see another member of your team on top of one of your buildings, that player is an enemy Spy! Even Engineer teammates can't stand on one another's buildings. Incidentally, it may be necessary to 'crouch jump' onto your own dispenser or sentry (done by hitting 'Ctrl' after jumping) to get on top of it.