What's the source of that map? At a first glance it seems wildly inaccurate. There are way more castles in France and Austria than listed here (and that are just the places I'm familiar with).

If this is some kind of crowd-managed map, much of the difference might be due to reporting bias.

Typing in 'castle' into Google maps is not going to get you an accurate picture. Try typing 'chateau' in and you will come up with the inverse. Try typing 'castle OR chateau' and you will get a more accurate (although obviously flawed) picture.

The history of the castle is fascinating and I'll discuss it in depth below, but the answer to your question is relatively simple - you're not using the right terms in a search!

Edit: Addition.

So while the question was answered relatively easily I’ll use this opportunity to explore why castles were so important and why they appeared so frequently in England.

Castles were more than just strongholds, although that was their primary function, they were expressions of power and represented one mechanism of control in medieval society. Here is how Abigail Wheatly described their importance in the opening of her study:

The castle had a dominant presence in medieval society, both physically and ideologically. Controlled by elites, castles towered over medieval villages and towns and were sites of judgement and administrative control.

Castles fixated and permeated medieval life. They appeared as decorations in heraldry, manuscripts, and even were depicted in pastry. They were used in sermons as allegories, and were the key locations of popular romance and gesta. Wheatly makes the adroit observation that the definition of castle is fluid and one that changes as new evidence comes to light; also that it is impossible to directly compare the authentic castles of medieval England to those represented in literature and art. [I’m a bit busy for the next couple of hours but I’ll try and re-visit this concept when I’m finished watching the football (come on Sunderland!)]

Curiously, the castle could be symbolic of royal authority or a lack of. To explain: castles tended to pop up whenever there was an individual with the power and capability to build one, in polities such as England and France you usually had to have the permission of the king (a grant ‘dono’ in charter form to build a castle).

This meant that in periods when the central authority of the monarch was negligible castles or towers tended to pop up across the country. For example, in England during the Anarchy of the twelfth-century the apparatus and authority of King Stephen had collapsed. We have fascinating documents which reveal how lateral alliances were formed to govern large swathes of England. The possession of, or right to build, castles was a key point of conflict and one which, during the Anarchy, led to intriguing bi-lateral feudal alliances, the agreement of the earls of Leicester and Chester highlights: neither may build any new castle ‘except with common consent of both. And if any one shall build a castle in the aforesaid places or within the aforesaid limits, each shall aid the other without ill will until the castle shall be destroyed’ (Stenton, 1932, 252). Upon taking the throne Henry II’s control over much of the royal demesne had been reduced, ‘through the weakness of King Stephen’ and his lords had profited from this decline. While the majority accepted Henry’s demands of repatriation of royal territories one, William earl of Aumale, ‘who had been the real king there [beyond the Humber] under Stephen’, refused until Henry physically proceeded north and William was finally forced to submit to ‘superior forces’. Among the royal possessions he return was ‘particularly that famous and noble castle called Scarborough’ (EHD, 1981, 349-350). These incidents highlight that the importance of castles and its centrality to proper governance in the twelfth-century. It was certainly a lesson that Henry II’s successors took to heart. While cumulative totals from pipe rolls of 1155-1215 show an average spending on royal castles ‘including those temporarily in the king’s hands’, of £780 p/a under John this rose to over £1,000 per annum (Eales, 1988, passim).

In France, Philip Augustus’s military exploits revolved largely around castles, ‘either as the objects of sieges or as the sites of skirmishes’ and his policy in Normandy centred on the seizure of fortresses and ‘to bring a policy of castle construction to perfection during the years 1203-1214’, it would lead to him possessing domainial rights for over one hundred castles. It should be acknowledged that investment in castle infrastructure was not solely a mechanism of control, it also could help insure that regions remained within the monarch’s sphere of control at all: Philip Augustus leant 4,000l. to the archbiship of Reims and 2,000l. to Châlons-sur-Marne, in 1209-1210, to strengthen the cities against the expected invasion of Flemish and Germans. (Baldwin, 1986, 295-301) Castles were also a powerful tool of patronage, Norman castles were utilised to reward loyal supporters, unlike Stephen, however, these were frequently held ‘on the condition that they furnish pledges to deliver the strongholds on royal demand’.

• John Baldwin, The Government of Philip Augustus, London, 1986.
• Richard Eales, ‘Castles and Politics in England, 1215-1224’, in Thirteenth Century England II, ed. P. Coss & S.D. Lloyd, Woodbridge, 1988, 23-44.
• Frank Stenton, The First Century of English Feudalism, Claredon Press: Oxford, 1932.
• Abigail Wheatly, The Idea of the Castle, York, 2004.
• William of Newburgh, ‘12. The History of England (c.1196)’ in, English Historical Documents: 1042-1189, Vol. 2, ed. D.C. Douglas and G.W. Greenaway, 2nd ed., London, 1981, 347-404.

Remember that you will get results in English while searching using English. For example, castles in france might be labeled as "château" rather than "castle", and would not be found using "castle" as a search term.

Spain has a ridiculous amount of castles built during the Reconquest period. The only way to control an area after having pushed the Moors back was to build a castle, so they are dotted all over the countryside.

I did some research, and it seems there is a lot of variance as to what you consider a castle these days. One in working condition? A ruin? A footprint?

That said, some estimates include:
1400 castles in England
1000 castles just in the Dordogne region of France 300 castles/châteaux in the Loire Valley
2500 castles in Switzerland 25,000 in Italy
and so forth.

Seems like it's a problem with OP's Google-fu and ethnocentrism rather than anything interesting with history.

Basically, this map shows the places in Europe where the local word for 'castle' is close to the English spelling.

France and Germany have more but you need to consider the spatial appearence as well. England is much smaller. France and England can both have 3,000 castles but on a map like that, it will look like England will have more because they are closer to each other. I have to agree with some of the other assessments though- this is probably not an accurate map and the language of the search won't pull up all of the French and German castles.

In a medieval history class I took last year, my professor talked to us about Viking (I think?) raiding parties who would come by sea, raid settlements, and then leave quickly via boats. Because it was so difficult to defend the entire coastline due to the raiding parties' elusiveness and ability to quickly leave, the society/societies of medieval England were forced to become much more centralized in order to defend. This might account for more castles than in the mainland states. Furthermore, the prevalent method of defence was to build small outposts along the coastline, which only housed small groups of combatants. Their primary task would be to burn/destroy the raiders' boats when they went inland to the raid, which effectively counteracted that strategy. Thus, more castles.

According to the ministery of culture, there are at least 11362 surviving castles in France. According to wikipedia, there are more than 600 castles in the departement of Dordogne (which is the department containing the most of them).
I fear google map is not a very precise tool to compare the number of castles from one country to another.

While we're at it, anybody know just how many castillos Castile actually had?

The german language has 2 words Schloss and Burg for the english word castle and on this map there is definitly only one of those used.