I'd like to propose also ruins=remnants instead of ruins=yes in case there is not much visible. It is suggested to use historic=archaeological_site in this case but it loses the information of the original building. The site_type proposal is not suitable, IMO. I'm interested in castles - usually, there are typical remnants of fortification, and calling one object historic=castle ruins=yes and another, which looks very similar at the first sight, but the walls already covered by soil, "archaeologic_site", seems to me a bit ... weird. Especially in the Czech Republic, where the hiking maps use the same symbol for both. --Kavol 22:23, 14 April 2010 (UTC)
Moved discussion here
I have transferred the following discussions here, in order to document actual usage
There is no wide consensus on using this tag, feel free to discuss on the page Proposed features/ruins for tagging ruins.
This was suggested on Proposed features/ruins#First solution