Complex sentence: Work on site began, and quickly ended in 1828, by the English mason João Johnston, who was responsible for the "English staircase", but was removed from the project mysteriously to work on the Palace of Nicolau Pires.
Split into simpler sentences: Work on site began, and quickly ended in 1828, by the English mason John Johnston. He was responsible for the "English staircase", but was mysteriously removed from the project to be replaced by Nicolau Pires.