There are quite a lot of pubs incorporating the name “Castle”, either that on its own or appended to the name of a place.1 One presumes the popularity of the name — the idea of the “castle” — comes from it being a fortress against the outside world, the province of lords and kings, an unreconstructed masculine space exerting authority over the world, not to mention a scene of carousing. I’m guessing at this, of course.
Perhaps this is also the reason there don’t seem to be quite so many really nice pubs by that name any more. In fact, a lot of my examples seem to be closed or renamed, though admittedly there are still a lot of pubs by that name I haven’t collected photographs of, which may disprove that perceived trend. A representative example of this may be The Castle (Camberwell SE5, now closed, fig. 31), where the name is an almost desperate bid by an estate pub to latch onto some grandeur in a egregiously run-down part of town. Its sign is a particularly fine example of wishful thinking.
My conceptual imagining of the meaning behind the “castle” extends to places named after a local street or area, the pub thus becoming the metaphoric castle ruling over that area, the modern equivalent of that mediaeval stronghold. So we have The Holloway Castle (now The Castle Bar, just off Holloway Road, Lower Holloway, N7), The Alwyne Castle (now The Alwyne, near Alwyne Square, Canonbury N1), or The Dover Castle (on Great Dover Street, Borough, SE1) — the latter also doubling as a real castle popular in pub names.
Where a specific castle is mentioned, there may be a more complicated origin. For example, The Dublin Castle (Camden Town NW1, fig. 32) may at first glance suggest the nationality of an earlier wave of immigrants to the area, surviving in several other Irish pubs nearby and throughout North-West London. However, a more interesting history lies behind it, as it was built during a period of intense railway construction — Camden Town lies just behind the major London termini of Euston, St Pancras and King’s Cross, where their many lines intersect and run through tunnels to the North — and the construction workers (navvies), sourced from all over the United Kingdom, were prone to factionalism. Hence the Dublin Castle served the Irish workers, while other pubs were built in other parts of the area (but not too close by) for the rest of the navvies. Thus there’s The Edinboro Castle (Camden Town NW1) for the Scots, as well as a Windsor Castle for the English and a Pembroke Castle for the Welsh.
References to royal castles, however, can suggest royalist sympathies amongst the drinkers, though these are perhaps not so strong now as in the past. There are plenty of Windsor Castles and even a (former) Balmoral Castle (now Kennedy’s, Lower Holloway N7).
A rarer case is The Baynard Castle (now the Cos Bar, Blackfriars EC4), which refers to an actual historic castle, Baynard’s Castle, which was sited in that area and which still lends its name to the local council Ward.2
But quite why such castles as Eastnor Castle (Somers Town NW1)3 or Thornbury Castle (Marylebone W1) are referenced in the names of pubs is a bit less obvious. One can only assume the original publican came from those respective parts of the country (Herefordshire and Gloucestershire in the examples given).
One Last Castle
History again informs the naming of Jack Straw’s Castle (Hampstead NW3, fig. 33), a former pub dominating the north-western corner of Hampstead Heath. Jack Straw was a fourteenth-century leader of the Peasant’s Revolt who was reputed to have fomented rebel sentiment by addressing groups on the local Heath.5 You won’t see much of that kind of behaviour in modern Hampstead, but then again nor are you likely to be able to take a drink in the pub that bears his name. Like many of the pictured examples, the modern “castle” is no longer a pub, but residential accommodation. The metaphorical has become literal: a man’s home is still his local pub.
See also: Search for pubs featuring the name “Castle” on my Flickr stream.
 This is quite apart from places called “The Elephant and Castle”, which I have no intention of discussing, nor of getting into the various possible etymologies of that name, most of them largely apocryphal. There’s certainly no persuasive evidence of the popular ‘Infanta y Castilla’ corruption.
 See the Wikipedia entry. It was destroyed in the Great Fire of London, 1666.
 See Wikipedia entry. It’s not even a real castle, but a 19th century impostor. For the Thornbury Castle, here’s a Wikipedia entry as well. You can see where I do a lot of my research, but I do have books as well.
 See Wikipedia entry, an attractive looking place, unlike the pub off Marylebone Road, a thoroughfare rarely described as particularly beautiful or pristine.