Why is there a cluster of tall buildings in the City of London?

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At present more than 60 City buildings exceed 75m in height and nine exceed 150m. Several more are on the drawing board. Most of the new batch are located inside an approximate triangle bounded by Liverpool Street station, Fenchurch Street station and Leadenhall Market. None are targeted for the western half of the City, nor anywhere near the river.

Why is there a cluster of tall buildings in the City of London?, in Diamond Geezer