In Cusco, altitude sickness can be a real problem. Casa San Blas provides incredible service and attention to help you remain safe and healthy. They have an oxygen tank you can use 24/7 (there is a night watchman who is very knowledgeable and helped us when we were having trouble with dyspnea) and provide free coca tea, always available for self-serve in the lobby.
Beyond this, Casa San Blas provided various other services that were really helpful to us. They helped us find a local pharmacy and advised us about ointment to treat mosquito bites from Machu Picchu. They also have a great breakfast service and free purified water (this saved us a lot of money). Every night in winter, they bring you a hot water bottle and each room has its own adjustable space heater.
The terrace on the top floor gives you a gorgeous view of the city at night and the street on which Casa San Blas is located leads you directly downhill to the main square.
We would recommend Casa San Blas to anyone traveling to Cusco. It is definitely the top choice of hotel here.