9. Grand Army Plaza Soldier and Sailor’s Arch
Unlike the Washington Square Park arch (also on this list), Grand Army Plaza‘s Soldiers and Sailors arch was designed with an observatory open to the public. Until 2008, the arch also housed the museum of The Puppet Library. There is a spiral staircase inside and visitors could wander freely to take in the museum. But when the arch started leaking, the space was closed off to the public and the museum had to find a new home (it’s now on the campus of Brooklyn College).
To add to the confusion about its public access, the Wikipedia entry on the arch says that it is open to the public, but this is definitely not the case.