Sorry for the poor wording in the title. English is not my native language
hi! there's lots of room for more input (Europe is a large area with many borders/frontiers and "before it was properly mapped" covers thousands of years), but you can get started on examples in these previous discussions
When did borders become rigid?
How well defined and enforced were national borders before the modern era?
How did Empires secure their borders while their army was away on campaign?