I just created a proposal page to tag citadels: Proposed features/Citadel - OpenStreetMap Wiki
The proposed tag is castle_type=citadel.
I’m sending this also to the tagging mailing list, but I prefer to discuss the proposal here and on the discussion page on the Wiki.
Voting is now open: Proposed_features/Citadel#Voting
I am pleased to announce that the proposal for castle_type=citadel has been approved with ten votes for, 1 vote against and 1 abstention.
My plan is to document the approved tag on the relevant Wiki page(s) and then I’m probably going to tag a few citadels with it.
My thanks to everyone who has voted and/or contributed to discussions on other communication platforms.