[citation needed] Rojas became a pupil of the king's engineer Tibúrcio Spannocchi, and became especially interested in military architecture.

After that he become very active as a military engineer, designing and building forts in Spanish territories during the Brittany Campaign, including in North Africa.

[2] Rojas undertook important works in Sanlúcar de Barrameda including the famous carved stone facada of the Santo Domingo Church in this city, in the mannerist style, started in 1596 and completed ten years later.

In his capacity as military architect and engineer he was appointed to rebuild and fortify the city of Cadiz after it had suffered a violent Anglo-Dutch assault and looting in 1596.

[1] Rojas's most important work was the overall fortification of the city of Cadiz, the most ambitious military work in the country during the reigns of Philip II of Spain and Philip III of Spain, including important elements such as the castle of Santa Catalina, started in 1598.