Oannes' Tower (a.k.a. Tower of Oannes) is an unreleased area. Some minor evidence remains of its existence, but it is confirmed to be in an unfinished state and not currently accessible. It is expected that this would be an extra area, similar to Hell Temple from La-Mulana, and may be added to the game in a later patch or released as DLC content.
Oannes' Tower Bonus Tour
Why yes, you did hear right - there are ruins dotting the area around La-Mulana. Sign up, and you can explore the area beyond the jungle on the village outskirts.
Note: The Oannes' Tower Bonus Tour humbly requests you bring your own shovel - you're going to need it.