The vast outer ring of bastion walls thrown up by Grand Master Nicolò Cotoner in the 1670s to defend the inland approaches to the Three Cities. The Lines run for about two and a half kilometres in a great arc, punctuated by eight bastions and pierced by the Notre Dame Gate (also known as the Żabbar Gate or Bieb is-Sultan) — a baroque triumph of military architecture with carved trophies above the arch.
You can walk most of the outer ditch and parts of the wall top without restriction. The Notre Dame Gate itself has been restored and houses a small interpretation centre. Best in late afternoon, when the limestone glows.