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Neither POI or area/multipolygon ruins are currently rendered. http://www.openstreetmap.org/note/114642 brought this to my attention, as the castle named in the note is already mapped as both POI and multipolygon.
For POI, perhaps just the name rendered, and for areas - well I'll let you pick a suitable colour. The wiki also suggests a lot of ways are also tagged as ruins - perhaps for these render a bit like generic barriers?
Yes, there is a need for this. historic=ruins should render on the map.