Can anyone guess what this small hole filled with water, in the center of a fort built in the 1600s on an island just off the coast of Colombia was used for?

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Based on hint 2 I'd say the level would rise when ships enter the bay. 

But for that, the fort has to be close to a bay. 

I can also see it being used for waterboarding of some sort.

Oh, wait.... No... It's messenger fish. Like pigeons, but the fish.
Right?

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I did say it was built on an island, in the Caribbean Sea, so not a bay exactly but… 😉

Answer:

Small narrow tunnels shot outward and connected to the sea. As boats approached they could tell which direction the boats were coming from by observing the water’s movement in the hole.

The local guides called it a sefon, and we made a joke it was a sea-phone, because it was a communication device for the sea. 

Pretty neat engineering design.

There were some good guesses in the replies.

Here’s the answer and here’s 500 $degen for taking a guess 🙏

Here’s the answer and here’s 500 $degen for taking a guess 🙏 

https://warpcast.com/adrienne/0xf90fc5b6

In my limited knowledge of that area, most of the forts were built to protect ports, that were often built in places that had some natural shelter. That's why I thought bay. Could be any indent on the coastline, to be honest. Might be my limited English vocabulary.