Wiang Kum Kam is the remains of an ancient city that preceded modern day Chiang Mai. It sat on the banks of the Ping river which frequently floods. This lead the city to eventually be abandoned around 700 years ago. At that time the city of Chiang Mai was established a little further from the river.
Chiang Mai has since expanded to encompass Wiang Kum Kam. Many more ruins exist than are visible but are hidden beneath people's houses and gardens. It's a shame they can't be seen but hopefully they are well preserved under there. There are still ruins to see.
Getting there can be a little tricky. Your best bet is getting a motorcycle or bicycle. It's on the east side of the Ping river, the opposite side from Chiang Mai's centre, south east of the old city. There are a number of ruins, some no more than foundations, scattered among what is now a residential area. It is not well signed and some back tracking will probably be involved before you find anything.
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When we visited there appeared to be people getting tours of the sites in horse drawn carriages. We don't have any details about costs or how you arrange this. It all seemed to be Thais getting a tour in Thai so I'm not even sure if English speaking guides are available.
We rode out to Wiang Kam Kam on our little motorcycle. If you do the same and you're extra lucky you may get the added bonus of a flat tyre. Our rear tyre went flat while we were on our way there. We stopped at a petrol station and, although they had no idea where we could get it fixed, they provided us with some air to buy us a little time. We then stopped at some guys sitting by the road who took us around the corner to a small workshop. There, a very kind man patched and refilled our tyre in about 3 minutes. Total cost was around a dollar. It seems like you're never far from help in Thailand.
There are no tickets to buy so, apart from the cost of your transport, Wiang Kum Kam is free. It's no Angkor Wat but, given that its 10 minutes from the centre of town, its worth popping out to see. Just be aware that Buddhist ceremonies are sometimes conducted at the old temples so dress appropriately in case you find yourself visiting the ruins with a group of monks.