This refers to the town walls of Pate Town on Pate Island, just off the coast of what is now Kenya. Pate Town, as the map on the Wikipedia page shows, is at the southwest tip of the island, by the sea:
https://en.wikipedia.org/wiki/Pate_Island

And as you can see on this map, the island is separated from Manda Island and from the mainland by a channel. The tourists sites say that if you're lucky you can see elephants swimming across this channel:
http://kenyapage.net/pictures/lamu.html

So I think that's what they're referring to: the town wall follows the coast of the island, that is, the channel.

I've found an article on the archaeology of Pate Island, though it doesn't really solve this, but it does indicate that there is a harbour at the west:

"The town walls, which were probably extant in some form in the sixteenth century, and were demolished, rebuilt and probably realigned several times over the next two centuries, are today mainly destroyed on thewestern, harbour, side, although elsewhere they are sometimes intact"
https://www.academia.edu/15087690/Archaeological_Investigati...