Complex sentence: The Romans built the "Limes Germanicus" to secure this border, i.e. a line of fortifications in regular distances, which included a small "castrum" on the site of the present-day Schirenhof field in Schwäbisch Gmünd.
Split into simpler sentences: The Romans built the "Limes Germanicus" to secure this border. Along the border they built fortifications in regular distances, which included a small "castrum" on the site of the Schirenhof farm in Schwäbisch Gmünd.