Too gigantick to fail

Corineus and Gogmagog were two brave giants who richly valued their honour and exerted their whole strength and force in the defence of their liberty and country; so the City of London, by placing these, their representatives in their Guildhall, emblematically declare, that they will, like mighty giants defend the honour of their country and liberties of this their City; which excels all others, as much as those huge giants exceed in stature the common bulk of mankind.

Thomas Boreman - The Gigantick History of the Two Famous Giants and Other Curiosities in Guildhall, London, via Faerie Lore

That's 'liberties' in the old-fashioned sense of the word:

Libertas in Medieval Latin conveys the idea of a right to exclude others from your property, your franchise. To be free of something is to enjoy exclusive rights and privileges in relation to it. The freedom of a town is a privilege, to be inherited or bought. So is a freehold estate.

Christopher Hill - The Century of Revolution 1603-1714.

But back to the giants themselves. Faerie Lore reveals their dark, gritty origins story:

The myth states that the Roman Emperor Diocletian had 33 wicked daughters whom he married off to 33 husbands who curbed their unsettling ways. However the daughters were so wicked, led by the eldest sister Alba, they plotted to cut the throats of their husbands as they slept. As punishment for this crime, they were set adrift in a boat with a half year’s rations of food, shunned forever. They drifted ashore the isles of what later became “Albion” (named after the eldest). Fornicating and coupling with demons, they populated the wild windswept island with a race of giants... When Brutus, great-grandson of Æneas, in company of his most able-bodied warrior Corineus, fled the fall of Troy, they by fate found themselves on these islands of Giants. Brutus was impressed with these isles so much that he named the Islands after himself, which later became called “Britain”. The leader of the Giants was a detestable monster named Goëmagot (Gogmagog), who stood in stature twelve cubits, and of such prodigious strength that at one shake he pulled up an oak as if it had been a hazel wand. Brutus and Corineus faced “Gogmagog“, had combat, and hurled him from a high rock to his death... As a reward for this defeat, Corineus was given the western part of the island, which many say is how Cornwall was called after him. After this defeat, Brutus travelled to the East and founded the city of New Troy, which eventually became known as “London”. [Geoffrey of Monmouth's 12th century Historia Regum Brittaniae]...
...Another mythos to their origins tell that the 33 infamous daughters of Diocletian who were captured and chained at the gates of Guildhall as guardians had given birth to numerous sons who were deemed to be “Giants”. The last two survivors of these offspring, were “Gog” and “Magog”. This comes from the lore around the carved giants guarding the gates of Guildhall during the reign of Henry V. They were added to the Lord Mayor’s Show in 1554 which were labelled in 1605 as Corineus and Gogmagog. 

Of course, some spoilsport had to come along and let the facts ruin a good story:

The tale chucks up more questions than I can answer. When did Gogmagog switch from one creature into two separate giants? ...We know it’s mostly toss because the fall of Troy was about 2300 years before the reign of Diocletian, and the name Gogmagog is a mangled borrowing from the Old Testament. But these histories were accepted as fact for centuries.

The history may have been toss but, sadly for the common bulk of humankind, fact has caught up with ahistorical fiction. The modern City and its privileges really are guarded by scary giants with a dodgy back story:

 As the Independent Commission on Banking led by Sir John Vickers noted, the assets of UK banks were nearly five times the size of GDP in 2009. In Germany and France, the figure was around three times, whilst in the US it was one-to-one.