“The long terrace blocks are grouped in meandering ‘U’ and ‘Z’ shapes on plan each enclosing, or otherwise defining, certain parts of the layout.”
Chamberlin, Powell & Bon, Architects “Barbican Redevelopment” April 1959
The architects were consciously emulating the effect of traditional London squares (even if the compliment is easily missed). They referred in their 1959 report to: “long terraces reminiscent of the London squares”. But they recognised that while providing a pleasantly enclosed space they had to avoid it being forbidding or isolating. They judged that the spaces between terraces were: “calculated to be in scale with – and pleasing to – people moving about them. Thus these spaces will not be so open that their form cannot be grasped nor so confined that the adjacent areas cannot be sensed.”
They needed to find a balance between claustrophobia and agoraphobia. You may think that the layout of the terraces is not particularly like London squares, but London squares are often anything but square. They were usually long runs of terraces built on a street grid which is roughly rectangular.
Most of the Georgian and Victorian squares which still stand were built on virgin farmland, so it was possible to create an efficient rectangular street design (as opposed to Chelsea and Mayfair where existing streets constrained later development). The general plan was to create terraces which were usually designed to seem one cohesive unit.
Sometimes similar terraces were built at right angles to seem to form a square. Of course there are also some squares which were genuinely built with shared corner properties, but you would never find a literal square, because no one would be able to drive in or out. So what you normally have is two or three terraces joined at rectangles. The Barbican repeats this plan quite literally. Defoe House, which is a terrace standing entirely on its own, is the exception in the Barbican not the rule.
Almost every other terrace is linked to a neighbour. John Trundle Court, Bunyan Court and Bryer Court form the closest thing to the traditional view of a London Square with their own semi-private garden in the middle (although Bryer Court is not physically joined to the others). These are an example of the U- shape, mentioned by Chamberlin Powell & Bon above. An example of the Z-shape is the group of Seddon, Thomas More and Mountjoy Houses.