Shouldn't the Pillar of Eliseg have a category of its own? It had been there for several hundred years when the Abbey was started nearby, but they're separate sites and events and involve separate people.
Of course, the Pillar of Eliseg is neither an abbey nor a castle, so it may need an over-category of its own as well! If you had a category for monuments, it could end up including every gravestone in Wales, so that could require some thought as well!