The term 'FORTIFIED FRONTIER' is a combination of two words, 'FORTIFIED' and 'FRONTIER'. The word 'FORTIFIED' means to strengthen or protect something, while the word 'FRONTIER' refers to an area that is on the edge of a settled area. The crossword solution for this term is 'Limes', which refers to a line of fortifications built by the Romans to defend their empire. These fortifications were made of stone, wood, and earth, and were often lined with lime to make them stronger. Therefore, the term 'FORTIFIED FRONTIER' can be used to describe a line of fortifications that have been strengthened or protected in some way.