[dancing spawn of ungoliant]

I think it's logical to make high buildings when living at the sea like the Valar and the Numenoreans at some point did. They didn't have light houses (except in Mordor in the 3rd age...ehh - sorry) but a high building can be seen quite far on a clear day like one can see far from a tower as well. It's also an advantage in war if the approaching enemy can be seen as early as possible.