Mexico City is the biggest city in the world, and the population is estimated to be close to 30 million people. It is spread across the Valley of Mexico, at the height of around 2,200m above sea level, and is surrounded by mountains. The traffic is unbelievable and the pollution reaches very high levels, but it is usually lowered by the showers that hit the city. The weather is mild, but locals say that summer is very rainy, so it is better to visit in the autumn or the spring.
Knowing that it was the biggest city ever, I thought that the city itself would be more high-rise, like other big cities I have visited before. In the center of Mexico City and in the zone of Santa Fe there are some higher buildings, but the city is dominated by low-rise buildings and there are, surprisingly, many trees and plants. One of the reasons for this is that in the huge earthquake of 1985, many buildings were ruined and were later rebuilt much lower.
There is a lot to see in Mexico City, but if you have limited time, I suggest that you take the Turibus and later explore more in detail the parts you liked most. Also, if you have time, it is well worth visiting the Pyramids of Teotihuacan and the small colonial town of Cuernavaca, which are both within a 1.5-hour distance by bus from Mexico City.