Many questions on this site mention how difficult it is for merfolk to develop technologically, as there's almost nothing but rocks and kelp to work with. My solution to this is that they use these rocks to build sea-spires that put them right in the center of maritime trade. These spires would essentially act as a port between continents for kingdoms lacking the economic might, navigational skills or shipbuilding know-how to sail all the way across the ocean. The towers would provide landmarks, trade, shore-leave, ship repair and other services on the jetties ringing the lighthouse like tower.
For what the tower is built of, materials acquired from the ocean or close to the shore are preferable. Merfolk can breath & walk, but to them it's like crawling is to us. This means they'll heavily prefer to trade for materials from further inland, making them more expensive and harder to acquire.
The deeper and wider the ocean, the more valuable these towers become but the harder they are to build. Can these towers get up and out to a useful height and depth or would the currents and weight topple them before they get out of site of shore?