Arrival from outside the Schengen Area is allowed in Monaco, but not in San Marino and the Vatican City as they have no border controls. Monaco is a part of the EU customs territory through an agreement with France, and is administered as part of France. read more
Border controls aren’t needed as France and Monaco are in a Customs Union and Monaco is treated as a de facto Schengen state) San Marino (Not EU - open border with Italy, no external border crossings, in a limited Customs Union with the EU, but the border remains fully open as taxation is broadly the same in San Marino and Italy. read more
In San Marino for example, foreigners staying longer than 30 days must apply for a residence permit and these are not given to visitors. But indeed, each day in a micro-state is a day out of the zone and the 90/180 rule is affected accordingly. read more