What saved Gondolin...

[Ithrynluin]

...from being discovered by Morgoth & co.?

Alright, the eagles are what comes to mind immediately. The servants of Manwë, mighty in their own right, but still...

Morgoth had quite a few winged servants. There were the dragons, extremely powerful and cunning. There were also some unclassified winged servants such as Thuringwethil.

And just for the sake of the argument, let's also assume that Balrogs did have wings.

Now, with all of these powerful servants, Morgoth could surely have espied the secret location of Gondolin? His servants didn't even have to come too close and engage the Eagles in combat, they could have simply soared very high in the sky - relatively close to the Echoriath but not too close - and seen everything around them.

Let's hear it!

 

[Celebthôl]

They all have poor eye-sight Therefore they have to keep far away to not get attacked by eagles, but they are still way to far away for their poor eye-sight to see much.

 

[Inderjit S]

The Winged Dragons only came into existence after the Fall of Gondolin.

Voronwe tells Tuor that no servant of Melkor had yet achieved flight in the high airs. If the Balrogs could fly it is most probable that they flew at a low altitude.

 

[Beleg]

The eagles had good eyesite. And they kept watch on Cirasgerim. So if any solitary winged servant might have tried to come closer the eagles would have espied it from afar and it chased it off. Morgoth wouldn't have been much worried by it because he would have taken for granted that eagles were doing so because they had their eyeries on the enciricling mountains.

 

[Ithrynluin]

Morgoth wouldn't have been much worried by it because he would have taken for granted that eagles were doing so because they had their eyeries on the enciricling mountains.

But perhaps it is possible that Morgoth hinted at the possibility that there was an Elvish realm 'there', since the Eagles of Manwë kept such vigil?

He was desperately looking for Gondolin, and nowhere else in Middle-Earth did the Eagles keep such steady watch, so perhaps Morgoth could have put 'one and one' together and figured it out? Though it probaly wasn't such an obvious or simple equation at all.

Also: How many entrances into Gondolin were there?

 

[Beleg]

But perhaps it is possible that Morgoth hinted at the possibility that there was an Elvish realm 'there', since the Eagles of Manwë kept such vigil?

He was desperately looking for Gondolin, and nowhere else in Middle-Earth did the Eagles keep such steady watch, so perhaps Morgoth could have put 'one and one' together and figured it out? Though it probaly wasn't such an obvious or simple equation at all.

And no where else in Middle-earth did eagles abode. Morgoth would know that Eagles have the special favour of Manwe and act as his messenger so they are bound to be hostile to him and his creatures. Therefore, once he had found out Enciricling mountains as Eagle's abode, he would have deemed their hostile behaviour and ceaseless watch as an item to protect their eyeries and as an act of hostility they bore towards Morgoth.

Which one's and one's? It was popular belief, or was bound to be a popular belief, that Turgon had developed his hidden Kingdom away in the south--In some forest or far off mountains. Morgoth didn't even knew the general direction of Gondolin till the Hurin facade. He didn't have a lot of clues...did he...he knew Turgon had disappeared. Did he knew the name of Gondolin before Nirnaeth? No.

Rumour came to Morgoth of these things, and he was unquiet amid his victories; and he desired greatly to learn tidings of Felagund and Turgon. For they had vanished out of knowledge, and yet were not dead; and he feared what they might yet accomplish against him. Of Nargothrond he knew indeed the name, but neither its place nor its strength; and of Gondolin he knew nothing, and the thought of Turgon troubled him the more..

Gondolin was totally hidden from him till Nirnaeth except for the subtle disappearence of Hurin and Huor from which he learned that they had the friendship of the King of Gondolin.

No one, not even the Noldor knew about where Turgon and his folks had went,

Then Ulmo returned to the sea, and Turgon sent forth all his people, even to a third part of the Noldor of Fingolfin's following, and a yet greater host of the Sindar; and they passed away, company by company, secretly, under the shadows of Ered Wethrin, and they came unseen to Gondolin, and none knew whither they had gone. And last of all Turgon arose, and went with his household silently through the hills, and passed the gates in the mountains, and they were shut behind him.

Through many long years none passed inward thereafter, save Húrin and Huor only; and the host of Turgon came never forth again until the Year of Lamentation after three hundred and fifty years and more.

Aredhel didn't even reveal to her Husband, Eol, the location of Gondolin.

As you yourself say it wasn't an obvious and simple equation. Probably no on except Ulmo knew about the hidden vale of Tumladen and besides no one would have thought that Turgon would build a secret refuge so close to Angband.

But offcourse once the general direction of Gondolin was revealed, the equation would have become easier and he would have put two and two togather and found out the real purpose of the eagle's vigilance.

Also: How many entrances into Gondolin were there?

The dry river bed. The secret refuge that Idril built and through air.

 

[Eledhwen]

Only treachery or torture could reveal the secret.

If Balrogs had functional wings, then Gandalf would not have been able to smite the ruin of his opponent at Moria.

 

[Ithrynluin]

Originally posted by Eledhwen
If Balrogs had functional wings, then Gandalf would not have been able to smite the ruin of his opponent at Moria.

The Balrog could have been extremely exhausted from the confrontation with another Maia. Birds can fall from the sky. But that is another matter.

 

[Eledhwen]

Touche! I think the wings were made of shadow type stuff, but such an arguments have been debated on the forum before and will be again - without resolution - because as far as I know, Tolkien never depicted a balrog in flight.

Gondolin remained hidden because they managed to give the impression that the city was somewhere else entirely. Just try looking for your wallet/phone/keys etc when you think you put them somewhere else than where you did.