I once bribed a goblin prison guard for details on the location of the castle. He told me to take my horse to the bottom of the mountain, go over the bubbling brook in the direction of the bells, carefully step over the woodsnake guardians (so as to not wake them), then down to the step right before the Bridge over the River of Lost Souls. The goblin did not know more, as they were blindfolded for the rest of their trip. I have marked the location of the bridge on my map with a [ ? ] Apparently the invisible castle is then within walking distance, if your armour is not too heavy.
To open the prison cell you will need some help. You will need to find 2 of the the following 3 trackables:
To determine the exact location of the prison, you'll need Grendl OR both keys. You'll also need to to take a key (either key will do) to the cache to unlock it. (You could unlock the prison without a key, with a large axe for example, but please don't do that.) The tracking codes on these trackables will allow you to find the secret number to whisper to the daughter of the late Supreme Wizard of Zertitude (click on the magic glyph below).
The secret number consists of all the numbers in Grendl's tracking code (eg if his code was A56B78 the secret number would be 5678). If you have both keys, the secret number is ALL the numbers on the tracking codes for the two keys multiplied together THEN multiplied again by the first number of Daphne's code THEN add 554 (eg if the tracking code for Daphne's Key was AB8C76 and the tracking code for the Swordkey of Love and Power was 4ABC51, then the secret number would be 8 x 7 x 6 x 4 x 5 x 1 x 8 + 554 = 54314).
You can validate your puzzle solution with certitude.
The First-To-Find will find a wooden FTF souvenir (for you to keep) and the imprisoned prince ( TB6F7WT ). If you choose to, you can rescue the prince and move him to a safe cache.
Know though that the prince, for his own good, needs to spend some quiet time in his prison every now and then. Or, to be frank, he goes crazy. If you meet him on the road, you might want to send him back to his prison! I know I will be on the lookout for him, if I find him escaped I will surely imprison him again.
Please ensure to replace the cache as found, and re-lock the prison.
I declare the following High Wizard of Central Coastia, for being the first to find this prison: LGDB
I declare the following Companions of the White Table, for rescuing the prince from his prison: LGDB, The GeoKrew, Wests Tigers Fan
I declare the following Companions of the Black Table, for placing the prince BACK into his prison: LGDB, Mumma-Kel
I declare the following Minstrels of Central Coastia, for finding this cache on the 30th or 31st of any month: mrnoo17, seaeagles1997, K1ngWizzzard, Purplepeopleater, Baktrak