'The Fortifications of London' are extensive and mostly well maintained. Today, with the threat of invasion a thing of the past, many of
Inner London's fortifications and defenses are tourist attractions, notably the
Tower of London.
History of London's fortifications
London's first defensive wall was built by the
Romans around 200 AD, 150 years after Londinium (as the city was called by them) was founded. There were seven main entrances through the wall into the City, five built by the Romans at different times in their occupation of London. They were the City Gates.
These 'gates' that once guarded the entrances to the City of London through the City Wall weren't gates as we might imagine them; they were multi-story buildings that had one or two archways through the middle for traffic, protected by gates and
portcullises. They were often used as prisons, or used to display dead bodies, or parts of dead bodies, to passers-by. Beheaded traitors often had their head stuck on a spike on
London Bridge, then their body quartered and spread among the gates.
After the curfew, rung by the bells of
St Mary le Bow and other churches at nine pm, or dusk, (whichever came earlier) the gates were shut. They reopened at sunrise, or six am, (whichever came later) next morning. Entry was forbidden during these times, along with everyone inside having to get off the streets and go home. The gates were also used as checkpoints, to check people entering the City, and to collect any tolls that were being charged for the upkeep of the wall, or any other purpose that might require money. It is possible that the wall was built for the sole purpose of collecting taxes, and not for defence at all.

The Tower in 1597
The gates were repaired and rebuilt many times throughout their existence. After the restoration of the Monarchy in 1660 all of the City gates were unhinged and had their portcullises wedged open, making them useless for defence. They were retained despite their uselessness for their original purpose because they were a visible sign of the prestige of the City. Most of them were finally demolished around a hundred years later in 1760 later due to a problem that still bothers us in the present - traffic congestion.
The positions of all the gates are now marked by a main road with the same name, except Cripplegate, which is a tiny street somewhat north of the position of the gate. The gates were:
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Aldgate
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Aldersgate
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Bishopsgate
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Cripplegate
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Ludgate
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Moorgate
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Newgate
List of fortifications
''this list is incomplete''
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London Wall
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Tower of London
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Montfichet's Castle
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Baynard's Castle
19th century
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London Defence Positions
World War II and later
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Outer London Defence Ring
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GHQ Line
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Cabinet War Rooms
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Military citadels under London
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Kingsway telephone exchange
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London deep-level shelters
References
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BBC - h2g2
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Museum of London Group
See also
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London
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History of London
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List of fortifications
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British hardened field defences of World War II