I agree that 'monumental protection' is wrong. (It would mean that the protection itself was somehow enormous, huge, imposing, monument-like.)
I've also seen 'listed' in BE, and can confirm that it's not AE.
I hadn't heard 'landmarked' before, but it seems to be in use at least in certain cities, perhaps especially for properties that have National Historic Landmark status, e.g., http://www.bostonpreservation.org/nhl.html
(though this designation apparently doesn't actually provide much legal protection).
I have the feeling, though, that US laws may vary pretty widely on this below the federal level, so that other descriptions might also be possible, like 'protected (as a) historic site' or 'designated (as) historic' or something. In general, we tend more toward designation than protection.