La Paz Bolivia is built in a canyon falling off the side of the Altiplano and at an elevation of about 12000 feet. If you want to ride flat terrain you can go up to El Alto at 13600 feet. Be advised that the area has historically suffered from oxygen scarcity.
Caracas is also situated in a narrow mountain valley. Oxygen is more plentiful but food is scarce.
Though these are impediments, cycling is still possible.