Without a doubt the most distinctive feature of St Martin's is the red and white stripy daymark at it's North East end, standing atop Chapel Down. The 11 meter tall structure, which can be seen from all around the scilly archipeligo is a rendered granite cylindrical tower with a conical top, which was constructed in 1683, in part to compliment the then new lighthouse built on St Agnes 3 years prior. The tower was built by Thomas Elkins, who encouraged resettlement on the island from the 1680s onwards.
The tower sits atop a 1.2 metre high rubble mound at the top of the hill, and at some point the date stone above the infilled doorway on the south-facing side was re-cut, providing the incorrect date of 1637. It is said that inside the tower there is a staircase, although there is no evidence to say that the daymark tower was ever intended for use as a lighthouse or lit beacon.
Nearby the tower are a ordnance survey trig point and prehistoric cairns, as well as the ruins of a napoleonic admiralty signaling station, which has earned the site Scheduled Ancient Monument status.