The city of Cusco is located at the altitude of 3400m and gives a great opportunity to feel all the charms of mountain sickness. Especially when arriving by plane from Lima (zero altitude).
Symptoms usually appear within the first four hours and last next day, sometimes a bit longer. Dizziness, lack of oxygen (hypoxia), insomnia, tachycardia and shortness of breath when walking up is the usually a light version.
But all these symptoms go away after a wile. To refrain from alcohol and smoking is crucial on such altitude. Oxygen cans don't help much, but chewing of coca leaves makes first days in the mountains easier. Look at local Indians - they always chewing them and feel happy.