There is unfortunately little left above ground of the massive fortress built by Roman Emperor Trajan (98-117) to protect a strategically important dock facility on the banks of the Nile. Several of the churches of Coptic Cairo are built on the foundations of the fort, and the Coptic Museum lies within its ancient walls. By 640, when the area was conquered by the Arabs, the fort had 40-foot walls and was linked to the Red Sea by a series of canals and interlinked waterways. Until fairly recently, much of the fort's foundations were flooded by rising groundwater, and they have only recently been dried out by a massive USAID-funded archaeological effort. It was unclear at the time of writing how much would ultimately be open to public exploration, but it's well worth taking some time while you're in the area to check it out.