http://wiki.openstreetmap.org/wiki/WikiProject_Fortifications
http://wiki.openstreetmap.org/wiki/Tag:historic%3Dcastle

possibles:
site_type=fortification
historic=castle +
castle_type=castrum
castle_type=defensive
castle_type=fortress
ruins=yes ?
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HB: for now I've used historic="pa", 'cause that's what it is and it is
rather more descriptive than historic="yes".
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RP: HB, yes, and i've added historic=pa to the wiki page
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HB: uploaded for the mainland Dec 13 2010 in changeset 6644528
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HB 22 Aug 2012: checked for new dataset, no changes.