It is an old city hall that has been converted into a museum to display historical stories, arts and culture from before the establishment of Chiang Mai to the present in a modern, easy-to-understand format with explanations in English for foreigners.
Inside, there are many exhibition rooms on a 2-story building with air conditioning every room, so you don't have to worry about hot or rain. You can walk around and enjoy yourself, and also get knowledge. The entrance fee is cheap. Worth it!.