Our guide for Prague, Kateřina Svobodová (here with Molly and Bob), met us all at the hotel the first day and immediately took us on our first outing to Vyšehrad. On the way, we got a quick lesson on Prague's mass transit system.

Vyšehrad means 'high castle' and this fits the description perfectly.  Perched on a promontory overlooking the Vltava (Moldau), this fortress absolutely controls traffic on the river below.  In an age of cannon, no one could sail past this point without permission.