This study demonstrates that a well-known drawing of a fortressvilla on a sheet in the Uffizi Collection, Florence, traditionally attributed to Francesco di Giorgio Martini, contains drawings by two different artists executed at different dates. A close examination of this drawing-namely, the ink used in drawing the bastions of the plan, a comparison between this ink and that used for the figure drawings which appear on the verso of the sheet-and a knowledge of Francesco's and Baldassare Peruzzi's military ideas, prove that Peruzzi was responsible for the later adaptation of Francesco's plan; it was Peruzzi who added the four triangular bastions to the original plan.