Why is Donato Bramante’s original design for the Belvedere Court difficult to recognize today?

Donato Bramante’s original design for the Belvedere Court is difficult to recognize today primarily because the project was never completed during his lifetime and underwent numerous modifications throughout the 16th century and beyond.1

The most significant alteration occurred in 1587, when the construction of the Vatican Apostolic Library across the center of the court destroyed Bramante's original unified concept. Because the scale of the project was so immense, it remained far from finished when Bramante died in 1514, leaving the complex vulnerable to the many changes that occurred as the work continued over the following centuries.21