The Pannonian Limes is a part that is mostly found in Hungary from the Roman front line known as the Danubian Limes, a line of castles and fortifications along the Danube River. The fortification line included guard towers, legionnaires camps (castra) and forts (castella).
The plaques are located in the March 15 square in Budapest where one of the camps stood, describing the relative location of the camps along the Danube which is also drawn on the sidewalk.
The current plaque shows Ad Flexum, the Roman name of the fort in Mosonmagyaróvár, Hungary. The fortress was built at the end of the first century AD, and survived until the beginning of the fifth century AD.
Notice the two lines representing the Danube River in the top-right corner of the image.
In the next photo taken on the same day, part of the line of fortresses was seen, with the route of the Danube River between the various plates Click for a larger image