Originally, these arches were part of walls, and they were literally gateways into the city. They have often been dismanted, moved and restored, giving the impression they were freestanding. If you look at classic paintings from the 1600 and 1700's done of many Roman ruins, you can see how they used to appear. Almost none of the 'ruins' of Rome and elsewhere are in their true ruined state. For instance, The Arch of Titus: https://i.pinimg.com/originals/a7/2b/2a/a72b2a99dc65259e11e5c5f4dd2c6d4e.png https://i.pinimg.com/736x/cf/94/dd/cf94dd7766485933e3e9367f1408ce8d.jpg https://i.pinimg.com/originals/77/ec/a9/77eca9325a338aff6677e69cee081e32.jpg https://cdn2.oceansbridge.com/2017/08/20101645/Arch-of-Titus-in-Rome-Clerisseau-Charles-Louis-oil-painting-1.jpg https://i.pinimg.com/originals/fb/7e/bf/fb7ebf157f0921b0eecd0810e440407c.jpg https://i.pinimg.com/originals/35/e7/ea/35e7ea90474349542f8b8a47888d43fd.jpg https://www.toperfect.com/pic/Oil%20Painting%20Masterpieces%20on%20Canvas/Stephan%20Bakalowicz_Polish_1857-1947/4-Arch-of-Titus-Stephan-Bakalowicz-Ancient-Rome.jpg https://i.pinimg.com/originals/a2/a0/4b/a2a04b4afece5cc96ff83fe9fc88f7d6.jpg https://i.pinimg.com/originals/0b/77/5c/0b775cbf75ddbbcc60b758f8b052b358.jpg https://www.allartclassic.com/img/Wittel_WIT003.jpg https://www.allpainter.com/gallery/giovanni-paolo-panini/giovanni-paolo-panini-architectural-capriccio-arch-of-titus-with-figures-241432.jpg https://upload.wikimedia.org/wikipedia/commons/1/17/The_Arch_of_Titus%2C_etching_by_Laura_Piranesi%2C_c._1780.jpg https://art.famsf.org/sites/default/files/artwork/rossini/5050161218010013.jpg http://www.rareoldprints.com/p/6078/$FILE/PICT0012.JPG