Do teleporters make for a good early warning system?
On CTF maps, here's a neat trick for getting spies. Note that this usually only works once, though...
- Place the teleporter entrance outside of your spawn or at another place a spy is likely to come across it.
- Place the exit down in the intel room. It MUST be placed either directly under or behind the intelligence.
- Build a sentry gun in the intel room, facing the exit.
- Wait
Spies will take the teleporter, assuming they'll come up somewhere close to where your stuff is built. Upon exiting the teleporter, the spy will unintentionally grab the intelligence, removing their disguise in the process. As a result, your sentry will blow them to kingdom come.
This is obviously very situational, but I've executed it several times on maps like 2fort and turbine. It is AWESOME watching spies rage at this trick.
This tactic has been used in the past, and savvy players know not to sap or attack teleporters when they're close to an objective.
If anything, this will provide an early warning to the savvy player that there is an Engineer nearby, and they will take extra precaution to prepare for a sentry gun.
Of course, there is always the chance that a not-so-savvy player will happen upon your teleporter and wreck it, giving you the warning you desire. But really, as engineer, how much warning do you need? The sentry is generally pretty good at picking off targets the instant it sees them, and it makes all sorts of racket alerting you to the danger.
Just stay alert, really. If anything, watch the kills at the top right; when your team has lost a lot of players recently, or if you see the name of someone you know to be a nearby defender, get ready.
Even if you don't plan on using teleporters, it's a good idea to place them.
For most classes, them taking out your TP won't make a difference, as StrixVaria noticed.
However, for whatever reason, most people who play Spy can't resist sapping teleporters. Sometimes, I even fall into this mind set, and I really should know better. Given that disguised Spies are the only enemy class that your sentry won't shoot, it's a good idea to know in advance that a Spy is around.
For the same reason, I will sometimes place a teleporter at one of our spawn doors even if I don't plan on using it, then use voice chat to warn my teammates if it's either sapped or destroyed as well as info on which class did it (note: This doesn't work if it was destroyed with a standard shotgun, as 4 classes use shotguns: Pyro, Soldier, Heavy, and Engineer). The other thing is that, if it isn't destroyed, I can later decide to use it by building an exit somewhere... but this removes your ability to tell where Spies are, because sapping one end of a teleporter saps both ends.
Other uses for teleporters
If you're defending an area in a map where the objectives only move in a single direction, consider hiding a teleporter exit somewhere the enemy may not look. As long as the entrance is not built, the exit will not spin up with flashy colors and noise, allowing you to build the entrance later once the activity has moved past where the exit is located.
pl_badwater is a good example for this. As RED, point 2 can be easily overwhelmed if you don't already have a defensive emplacement there. However, if BLU does quickly take point 1, they may not check the back area behind point 2 as well as they should. Particularly since the door between point 1 and point 2's back area is closed prior to point 1 being taken.
pl_frontier_final is also an interesting example. There is a back route between points 2 and 3 that is not commonly used. It is too high for RED to normally jump up to it, but putting a Dispenser down can allow you to jump up to it (just make sure you pick up or destroy it. Using this passage, you can sneak back to point 1 and place a teleporter exit, allowing you to move troops behind them.