With the background of Snowdonia we see here the smaller tower on this Island. It is in fact the very old Tŵr Bach Lighthouse
Somewhat overshadowed by Tŵr Mawr — a fact that’s also backed up by it’s name, meaning ‘small tower’ —Tŵr Bach Lighthouse was actually the original lighthouse on Llanddwyn Island. It is believed to have been built as a landmark and day beacon (an unlit lighthouse). However, it couldn’t easily be seen by ships approaching from the west, and for that reason, another lighthouse (Tŵr Mawr) was built in its place.

See PiP for Tŵr Mawr