Why was the site of San Carlo alle Quattro Fontane difficult to build on?

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Rome: Piazza Navona — Francesco Borromini Piazza Navona, Rome, with the church of Sant'Agnese in Agone, designed by Francesco Borromini, and (foreground) the Fountain of the Moor, originally designed by Giacomo della Porta and revised by Gian Lorenzo Bernini.

The site of San Carlo alle Quattro Fontane was considered difficult to build on because it was extremely cramped and narrow.12

Architect Francesco Borromini was tasked with fitting a complex, influential Baroque church into this restricted space, which was no larger than the dimensions of a single pier at St. Peter’s Basilica. To overcome these spatial limitations, Borromini utilized an unusual, irregular floor plan based on a Greek cross defined by convex curves and employed innovative geometric configurations to integrate the structure’s various elements, including its dome.132