Flown over Mexico into Monterrey many times. You have to be very careful with their ATC and clearances, especially altitude assignments. Monterrey airport surrounded by high jagged mountains. We were required by Co procedures to respect the altitudes on the charts and to stay on the DME arcs inbound to the airport. That arc kept you east of the mountain range, and on the rare day when the smog wasn't bad you could see how those jagged mountains looked, and they looked very unforgiving.