Abu Simbel is a spectacular looking site located in the most Southern part of Egypt and in very close proximity to the Sudan-(Ancient Nubia). Although Abu Simbel was a massive Temple dedicated to Pharaoh Ramses II, the Temple's facade has 4 enormous statues overlooking the Egyptian-Nubian panorama. Having said that, was this Temple also a type of ancient border signalling to the Nubians-(as well as to any visitor(s) from lands South of Nubia) that they were entering Egyptian territory?
It doesn't seem so.
Earlier Egyptian temples in Nubia, for example the temple of Wadi al-Sabua, had been located within what appear to be fortified enclosures.
That does not appear to be the case with the Abu Simbel temples. This, in turn, suggests that by the time these temples were being built, they were considered to be safely within "Egyptian" territory, and did not require fortification.
A useful source here is Richard Wilkinson's The Complete Temples of Ancient Egypt.
This article on Egypt and Nubia describes the relations between the two countries over time, and details Egypt's military expansion into Nubia in the New Kingdom. It shows that Egypt's southern border was far to the south of Abu Simbel when the temples were constructed.