Yes that's it.

Very good point Sam. That tunnel is clearly a legitimate location for sentries (usually multiple sentries), and Spies don't have a chance of getting to them, it's down to good 'ol firepower to take them down.

A spy can sap them though, jus jump and click, simple as that

Yup if they are in the best location up there a spy can sap them, if they are anywhere else they cant see all of the corner and so are useless anyway so it matters not if they cannot be sapped because they will last seconds :f

I'm with fjack and Agent 47 on this one i.e. anywhere on a map and not an exploit, being legal.

They can still see some of the corner though and kill people who walk past.
Also a teleporter can be up there right at the back of the tunnel so it can't be sapped, yet still be very useful.

I'd also like to know what the official stance is on spawncamping. Different admins have different thoughts on it. I don't see a problem with it; unlike other games the other team don't die as soon as they spawn, they have plenty of time to prepare themselves in their spawnroom (which contains a resupply cabinet) and communicate with their teammembers to beat the spawncamper.

I agree that part of dustbowl is an oddity but it is controlled by the first cap and once access via the door is denied you are trapped in there, if you put a sentry at the back to guard the teles once into the 2nd cp stage then it is only doing that and it could be far more useful elsewhere imo. I say it's an oddity because if you take goldrush stage3 red have almost the same settup with the quick route to the first checkpoint but the ledge isn't buildable on and once the first stage is over the outer door closes when no reds remain in there, so totally contrary to what happens in dustbowl, will valve alter dustbowl to match I wonder?

Okay, I'm going to grind it down into pure logic.
If you can get there by any means necessary, it's legit. It's called being creative. Aslong as you don't glitch the map by putting it in walls, the sky, underground etc, any place that you can't actually get to under any circumstance then it's bannable.

Lets take the small balcony up on the last cap on Goldrush stage 1, I got up there. I placed a dispenser there, got a teleport up, and my sentry. Managed to upgrade my sentry in time and I had a Heavy standing there milking my dispenser to guard me aswell. I got about 4 kills and then my sentry was taken down in a matter of a minute because despite the position, my sentry doesn't reach everywhere. You can take the B Roof on Gravelpit as a perfect example of why it should be allowed.

Me being a proffesional spy, I don't care if I can't get up there because I have an entire team to kill that sentry for me and I have other sentries to deal with meanwhile.

To sum it up, if it's destroyable then it's legit.

Another reason is that VALVe know those spots are there, and if they want to fix it or not is none of my concern, but until they do it's fair game because I haven't been told it's against the rules. It's simply being smart with your buildings to reach certain places.

I saw this thread when DJ asked the question and was really confused; I thought everywhere was fair game (obviously except spawn doors imo). I honestly hadn't thought that anywhere would be considered illegal.

Can I ask what prompted this topic DJ?

There's been a couple of times on the servers where ppl have built on one of the spots I mentioned only to be told by an admin that it was an exploit and not allowed on the servers.

Most of the time though when it happens nothing is said.

I personally think if the sentry is destroyable, its fair enough. Spies not being able to access them isn't enough of a reason to prohibit them imo. Its not like spies are the primary class for taking down SGs.

The spots in question don't really give an undue advantage (apart from maybe the surprise factor, and that doesn't last long).

Engineer was my most played class when I started playing but I soon got bored of it as there are a very limited number of effective sentry spots (at least on defence). Allowing these more unusual spots gives an engie more room to be creative, which I think can only be a good thing.