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How did Leonardo da Vinci serve Cesare Borgia as a military engineer?

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Self-portrait of Leonardo da Vinci drawn in red chalk, showing an elderly man with long hair and a flowing beard, with a serious, contemplative expression. — Leonardo da Vinci Self-portrait, drawing by Leonardo da Vinci, c. 1517–18; in the Royal Library, Turin, Italy.

In the summer of 1502, Leonardo da Vinci entered the service of Cesare Borgia, the commander in chief of the papal army, serving as his "senior military architect and general engineer" and inspector of fortresses.12

During his 10-month tenure, Leonardo traveled extensively across the territories controlled by the condottiere to survey the land. His primary contributions involved:1

Cartography: He created topographical maps and city plans, which are considered early examples of modern cartography.1
Strategic Surveying: By traveling through the Romagna and the Marches, he provided the technical expertise necessary for Borgia to manage and secure his expanding territories.1

While Leonardo was deeply involved in these military and engineering tasks, he did not execute any artistic commissions for Borgia during this period. His time in Borgia's court also brought him into contact with Niccolò Machiavelli, who was observing the political climate of the region.21