It is often said that the Irish border is the only land border that the United Kingdom will share with the EU after Brexit takes place.
This is correct.
However, it is not the only land border/boundary that an EU Member State will have with an area that has a constitutional relationship with the UK. Indeed, Gibraltar and the UK Sovereign Base Areas in Cyprus will share a territorial border/boundary with Spain and the Republic of Cyprus respectively. This is something that the Withdrawal Agreement has recognised. Apart from the Protocol on Ireland/Northern Ireland that contains the infamous backstop, there are two other protocols on SBAs and Gibraltar.
Together with my colleague Nasia Hadjigeorgiou, we have authored a brief piece analysing the foundational documents that led to the establishment of the SBAs; their status in EU law after the Republic of Cyprus acceded to the EU; and finally how the UK Withdrawal Agreement accommodates them post-Brexit.
You can download it from here.