Frodo? Wrong book, man...
The guard tower was to keep a watch on Mordor, and it was built by Isildur. What was his brother's name and city?
Duh! Of COURSE it was Bilbo. That was just a brain fart on my part.
Isildur's brother was Anarion. Their city would have been Osgiliath, yes? Because Minas Anor and Minas Ithil were just outpost fortresses when Osgiliath was at its height.
I confess, my knowledge is sketchy on those points.
Oh, a question...Who was it that proposed the riddle game in the chapter of The Hobbit titled "Riddles in the Dark"?