From OpenStreetMap Wiki
|defensive = keep|
|Fortified residential tower of a castle|
|Used on these elements|
|Tools for this tag|
(en) is the main tower of medieval castle in countries under English influence. It has both residential and defence function. Its continental counterpart is the French Donjon.
How to map
You can use the tag building:condition=state to indicate the condition of the tower.
If it is ruined, add the tag ruins=yes.