Yes the Uboat pens are impressive.  I visited them in the 80s and according to my French brother in law one of the reasons they are stilll there is that the Council can't find any way of demolishing them safely!

Lost this thread, until some kind person put it into The Bunker

I've been hanging on to these links, which don't feature the flakturm.

The following concentrates on the more normal air-raid shelters of Germany, and possibly the most significant feature about these is their overground construction, which is clearly in contrast to the British preference for digging a hole in the ground:

German Air-Raid Shelters - an introduction to the Luftschutzbunker of World War II

I tried to visit the site you mentioned here about Hiler's bunkers but couldn't get into it. Anyway I did extract a site from the wording and found it very interesting. I even found a picture of memorial to those who died working at the REHMAG underground Me-262 factory!

With regard to air-raid shelters, I'd guess the overwhelmingly-vast majority of Glasgow's shelters were not underground but rather , above ground. One need only consider the number of street shelters, backyard shelters (not anderson type), reinforced closes.On the other hand, while Germany had some superb shelters above ground, I'd guess the majority of theirs was partially underground in houses, anmd apartmant block cellars for example (I'm thinking here about Hamburg and Berlin in particular).

The gigantic flak towers, which were absolutely superb structures, were first and foremost flak towers, their role as air-raid shelters was very much a secondary role. I could be wrong, but I don't think the Allies ever succeeded in destroying a flak tower. They provided a formidable shield for the cities they were guarding, and a high price was extracted for the aircraft that did successfully get through to their target area. I cannot imagine the rate of  fire which came from the heavy 128mm guns on the top of these towers . Russian POW's were actually used as part of these tower gun crews, and according to what i read they did a great job... the crews competed with each other to see who could fire the greatest number of shells and the winner got a bottle of Champagne!

Sorry, I was/am having odd problems with ghost keystrokes - link is fixed now

I agree about the shelters, I should probably have expanded a bit on the comment I made, as what was in my mind at the time was the government's directive that things like underground stations and the like were not to be used by the public as air-raid shelters, but the public just went ahead and used them, and the government had to withdraw the rule.

I think I mentioned that the Flakturm were almost completely avoided by the Allies, who flew around them rather than try and attack or pass them, such was the rate of fire - as you note. The Allies never destroyed one (I don't even think they tried), and even the Russians (I think) made little impression when they attacked with ground based artillery - I'm sure there's mention somewhere of the shells doing little damage, such was the thickness of those walls.

I revisited the site you recommended Apollo, and I'm glad i did. This site is about "Turms" that are were used only as air-raid shelters, and i hadn't known such buildings had existed. I thought all these towers were anti-aircraft towers, which could also accommodate some civilians. I certainly don't ever recall any such structures in the UK, and yes, as I say, hadn't even heard of them in Germany. Mind you, I must say, they were depressing looking structures! Interesting stuff.