'Tranquilo, tranquilo!' says the gentle concierge with a beaming smile and relaxed demeanour. 'OK' I retort nervously, 'but there are 1,000 police surrounding the square in riot gear. Is there a problem?' 'No problem, no problem - tranquilo, tranquilo!' he repeats with just as much enthusiasm and some amusement at my anxiety. Mexico city, June 2015. A remarkable city in a poor country - steeped in magnificent history, surrounded by incredible beauty, impacted by the system - poverty, drugs and corruption.