镇(zhèn) - town, small town in Mandarin

镇 (zhèn) is a town: bigger than a village, smaller than a city, and in mainland China an actual level of local government sitting below the county. 小镇 is the everyday way to say a small town.

The word takes 上 rather than 里 for most locational uses. 镇上有一家医院 is the natural phrasing, in the same way English says "in town" rather than "inside the town". Learners who reach for 在镇里 are understood but sound translated.

The character has a second life as a verb meaning to hold down or to calm: 镇静 is composure, 镇痛 is pain relief, and a 镇 was originally a garrison posted to keep a district quiet. That is the thread back to the metal radical.