Cusco was the capital of the Inca Empire and now serves as the gateway to the majority of touristy type activities you're likely to choose to do in Peru.
Flying to Cusco from Lima, you'll pass over some impressive mountainous and rugged terrain and drop into the city which sits at the heady elevation of 10,912 feet above sea level (3326m). The altitude acclimatisation starts with a vengeance as you step off the plane and you can really feel the lack of oxygen slowing down any strenuous urges that you may have.
The city is very laid back, rustic, and has a great safe touristy feel to it. The only real annoyance is the number of touts lingering on the streets trying to get you to eat in their affiliated restaurants or peddling their wares. The phrase 'no gracias' ends up being used a lot ... in fact it'd be quicker to write it in marker pen on the palm of your hand and just issue it abruptly to any of them targeting you.
Local to Cusco are many Inca ruins, including the most famous 'Saqsaywaman' which really shows off the Inca stoneworking skills. The size of the foundation stones and the custom cut of each piece defies belief.