城(chéng) - city, town; city wall in Mandarin

城 (chéng) is the single-character word for a city, and it originally meant the wall around one: the earth radical is there because ancient city walls were rammed earth. The city and its defences were the same thing.

On its own it survives mostly in set phrases and compounds: 进城 (jìn chéng, to go into town), 全城 (quán chéng, the whole city), 城里 (chéng lǐ, in town), 长城 (Chángchéng, the Great Wall), 城市 (chéngshì, city). For a plain noun in a modern sentence, most speakers say 城市.

The measure word is 座 (zuò), the one used for large fixed structures, which is a nice reminder of where the word came from.