Unlike many of his predecessors, Montiano had the ear of the Cuban Governor. Guns and men came from Havana. There was money to strengthen the fortifications and in the summer of 1738 began the work of tearing down the old rooms inside the fort and laying foundations for the 28 great arches that were to make the new rooms proof against English bombs. While the carpenters were setting up the forms for the arches, while the quarries and the limekilns were again the scenes of feverish activity, James Oglethorpe in his buffer colony of Georgia was growing stronger and stronger, pushing the Florida boundary ever closer to the St. Johns River--a scant 35 miles north of the castillo.
Then the ponderous arches were finished and hurriedly leveled off with a packed fill of coquina chippings, sand, and shell. Hundreds of bushels of lime went into the tabby or mortar that was spread over the entire roof of the renovated fort to make its terreplein. The tampers beat the wet mixture smooth, and when the first layer was hardened, another and another was added until there was a bed of tabby 6 inches deep. Upon this smooth, hard surface the cannoneers could maneuver their heavy guns and the rooms below were safe under 2-1/2 feet or more of solid masonry; in fact, on the eastern side, where heavy bombardment was most likely, the engineer allowed a minimum thickness of 4 feet. Some of the parapets had to be rebuilt for modernization. Outside the fort a new stockade was erected to strengthen the covered way, and the walls enclosing the town were reworked. Under Montiano's dynamic leadership and the able supervision of Engineer Pedro Ruíz de Olano, the work was practically finished by 1740. There was no time to spare.
The War of Jenkins' Ear precipitated Oglethorpe's invasion of Florida. When the first English warship appeared off the bar of St. Augustine in June (by the Spanish calendar) of 1740, Montiano hastily sent the news to Havana: here was the long-expected Siege of St. Augustine. Reënforcements had brought the 350-man garrison up to about 750 against General Oglethorpe's force of about 900 soldiers, sailors, and Indians. Oglethorpe landed his guns across the bay from the fort, and as British shells began to burst over the town, the inhabitants, almost 2,000 of them, fled to the fort. "It is impossible," wrote Montiano to the Governor of Cuba, "to express the confusion of this place ... though nothing gives me anxiety but the want of provisions, and if Your Excellency ... cannot send relief, we must all indubitably perish." There was no hint of surrender.