Adulis was once the primary port of the Aksumite empire and a few impressive architectural remnants of this heritage remain. Most notable is the foundation of a large 5th-century Byzantine basilica. Archaeological work at Adulis (begun in 1840!) is ongoing, but moves slowly for a month or two each summer.
Local Saho tribal legends say that the name 'Adulis' originates from the words for 'white water', a testament to the settlement's proximity to the Red Sea and the Aksumite-era prosperity as a major port and trade link between the Roman empire and India. Aside from the basilica, most of the ruins are still quite buried and a healthy dose of imagination is necessary to make the most of a visit. All of the artefacts that have been recovered from the site are currently housed at museums in Asmara and Ethiopia, though look for a large pottery dump southeast of a large warren of excavated walls the original function of which are still unknown. Access to the site is free, but the suggested tip for the mandatory local guide is around nfa400.
It's around 50km from the mainland side of the causeway in Massawa to the village of Faro, which is the last spot to stock up on water or food. From Faro follow the highway until it descends to the river, then turn left into the desert on an extremely rough and poorly marked desert road.
A few kilometres south of the Adulis site is Zula village, full of traditional Tigre ethnic group houses and a 150-year-old well built by the Ottomans. It's worth stopping by on your way to or from the ruins.