Same for France and Spain, which is what I am familiar with. I am expecting it to be true all around.

The map of France shows maybe 100 castles? If you include castles, forts and chateaux (which it seems the map aims to do) France has around 45000 of them, including 15k medieval castles (military), an other 15k from the Renaissance period (mix of military and luxury), and 20k more recent ones (some military, many luxury type).

I know if at least 20 medieval castles within 1h of my hometown and not a single one is here.

For Spain maybe 10k to 20k castles and forts. Out of which at least 2500 genuine medieval castles (not just big houses with fortifications)

Missing all of the "moorish" forts in Andalusia...

and for Romania, which has far, far more than are listed

and Poland....

For England as well, assuming a conservative definition like: Norman or later, mortared stone construction with remaining walls at least 4m high, not something like a fortified farm house for a single family. If you include Iron Age hill forts, the number becomes absurd.

Seems like your map lists pre-historic ring forts, not castles?

I'm not sure what the 'official' definition of a castle is, but I personally wouldn't consider a crannóg or something like Dún Aonghasa a castle. Nor would I consider a stand alone tower house a castle, although they might (along with gate lodges and great houses), sometimes be colloquially referred to as castles.

Surely a castle is a fortified building, specific to the middle ages. Wikipedia agrees - https://en.wikipedia.org/wiki/Castle

Which would put the number of Anglo-Norman castles in Ireland much lower, probably something around 100.

Yeah, in the other direction, I see at least one WWII-era "pillbox" bunker in France (part of the Marginot Line?) listed as "fortress".

It's a nice idea, but the range seems extremely broad on the one hand but very incomplete on the other hand.

I like the definition where castle is a fortress where a Nobel lives. Fort is where an army lives but no Nobel leader. Palace is where a Nobel lives but it isn't expected to withstand a siege

*Noble. Alfred Nobel invented stable explosives and started a Norwegian prize for achievement in science. Nobles, however, were high ranked members of the feudal hierarchy, often passing their position on to their children.

Wasn’t he Swedish?

Yes. All the prizes except the peace prize are awarded in Sweden. Norway was in a union with Sweden at the time the prizes were set up, but it's not definitely known why he chose the Norwegian parliament (via the Norwegian Nobel Committee) to aware the peace prize.

In the intro paragraph of the linked Wikipedia page: “Use of the term has varied over time and, sometimes, has also been applied to structures such as hill forts”.

The map has a category for fortresses too. Which are included in its total count of "castles". So it's seems reasonable to bring them up.

Fair, I think the map is nonsense, but also the idea of 30K castles in Ireland is clearly untrue.

Same for Scotland - there are 3 castles within 2 km of where I live, not including a couple of really old ruined hill-forts... which I don't think is an unusual density of these things for this part of the world.

Edit: And of course there is also the huge castle 13km away across the sea sitting on its volcanic plug and very visible from here.

Same for France, many are missing