Before the Castillo was completed, guns were mounted only in the bastions or projecting corners of the fort. A 1683 inventory clearly shows that heaviest guns were in the San Agustín, or southeastern bastion, commanding not only the harbor and its entrance but the town of St. Augustine as well San Pablo, the northwestern bastion, overlooked the land approach to the Castillo and the town gate; and, though its armament was lighter, it was almost as numerous as that in San Agustín. Bastion San Pedro to the southwest was within the town limits, and its few light guns were a reserve for San Pablo. The watchtower bastion of San Carlos overlooked the northern marshland and the harbor; its armament was likewise small. The following list details the variety and location of the ordnance:

Cannon mounted at Castillo de San Marcos in 1683

LocationNo.CaliberClassMetalRemarks
In the bastion of San Agustín 1 40-pounder Cannon Bronze Carriage battered.
1 18-pounder do do New carriage.
2 16-pounder do Iron Old carriages, wheels bad.
1 12-pounder do Bronze New carriage.
1 12-pounder do Iron do.
1 8-pounder do Bronze Old carriage.
1 7-pounder do Iron Carriage bad.
1 4-pounder do do New carriage.
1 3-pounder do Bronze do.
In the bastion of San Pablo 1 16-pounder Demicannon Iron Old carriage.
1 10-pounder Demiculverin Bronze do.
2 9-pounder Cannon Iron do.
1 7-pounder Demiculverin Bronze do.
1 7-pounder Cannon Iron Carriage bad.
1 5-pounder do do New carriage.
In the bastion of San Pedro 1 9-pounder Cannon Iron Old carriage.
2 7-pounder do do Carriage bad.
2 5-pounder do do do.
1 4-pounder do Bronze Old carriage.
In the bastion of San Carlos 1 10-pounder Cannon Iron Old carriage.
1 5-pounder do do New carriage.
1 5-pounder do Bronze Good carriage.
1 2-pounder do Iron New carriage.

The total number of Castillo guns in service at this date was 27, but there were close to a dozen unmounted pieces on hand, including a pair of pedreros. The armament was gradually increased to 70-odd guns as construction work on the fort made additional space available, and as other factors warranted more ordnance. Below is a summary of Castillo armament through the years:

Armament of Castillo de San Marcos, 1683-1834

Kind of gun1683170617401763176518121834
IronBronzeIronBronzeIronBronzeIronBronzeIronBronzeIronBronzeIronBronze
2-pounder 1 .. .. 8 guns from 2- to 16- pounders .. .. .. .. .. .. .. .. .. ..
3-pounder .. 1 .. 2 3 .. .. .. .. .. .. .. ..
4-pounder 1 1 26 guns from 4- to 10- pounders 5 1 .. .. .. .. 1 .. .. ..
5-pounder 4 1 15 1 .. .. .. .. .. .. .. ..
6-pounder .. .. 5 .. .. .. .. 1 .. .. 3 ..
7-pounder 4 1 5 2 .. .. .. .. .. .. .. ..
8-pounder .. 1 11 1 5 11 .. .. 1 .. .. ..
3-1/2 in. carronade .. .. .. .. .. .. .. .. 4 .. .. ..
9-pounder 3 .. .. .. .. .. .. .. .. .. .. ..
10-pounder 1 1 .. .. 6 .. .. .. .. .. .. ..
12-pounder 1 1 .. .. .. 13 .. 7 .. 2 .. .. ..
15-pounder .. .. .. 6 .. .. .. .. .. .. .. .. ..
16-pounder 3 .. .. .. .. 2 1 .. .. 8 .. .. ..
18-pounder .. 1 .. .. 4 1 7 .. .. .. .. .. 4 ..
24-pounder .. .. .. .. 2 .. 7 .. 32 .. 10 .. 5 ..
33-pounder .. .. .. .. .. 1 .. .. .. .. .. .. .. ..
36-pounder .. .. .. 1 .. .. .. 1 .. .. .. .. .. ..
40-pounder .. 1 .. .. .. .. .. .. .. .. .. .. .. ..
24-pounder field howitzer .. .. .. .. .. .. .. .. .. .. .. .. 2 2
6-in. howitzer .. .. .. .. .. .. .. .. .. .. .. 2 .. 2
8-in. howitzer .. .. .. .. .. .. .. .. .. 2 .. .. .. ..
Small mortar .. .. .. .. .. .. .. 18 .. 20 .. .. .. ..
6-in. mortar .. .. .. .. .. .. .. .. .. .. .. 1 .. 1
10-in. mortar .. .. .. .. .. .. .. .. .. .. .. .. .. 1
Large mortar .. .. .. .. .. .. .. 6 .. 1 .. .. .. ..
Large mortar 2 .. .. .. .. .. .. .. .. .. .. 3 .. ..
Total 20 9 26 9 55 10 40 37 39 24 26 8 14 6
Grand total 29 35 65 77 63 34 20

This tabulation reflects contemporary conditions quite clearly. The most serious invasions of Spanish Florida took place during the first half of the eighteenth century, precisely the time when the Castillo armament was strongest. While most of the guns were in battery condition, the table does have some pieces rated only fair and may also include a few unserviceables. Colonial isolation meant that ordnance often served longer than the normal 1,200-round life of an iron piece. A usual failure was the development of cracks around the vent or in the bore. Sometimes a muzzle blew off. The worst casualties of the 1702 siege came from the bursting of an iron 16-pounder which killed four and seriously wounded six men. At that period, incidentally, culverins were the only guns with the range to reach the harbor bar some 3,000 yards away.

Although when the Spanish left Florida to Britain in 1763 they took serviceable cannon with them, two guns at Castillo de San Marcos National Monument today appear to be seventeenth century Spanish pieces. Most of the 24- and 32-pounder garrison cannon, however, are English-founded, after the Armstrong specifications of the 1730's, and were part of the British armament during the 1760's. Amidst the general confusion and shipping troubles that attended the British evacuation in 1784, some ordnance seems to have been left behind, to remain part of the defenses until the cession to the United States in 1821.

The Castillo also has some interesting United States guns, including a pair of early 24-pounder iron field howitzers (c. 1777-1812). During the 1840's the United States modernized Castillo defenses by constructing a water battery in the moat behind the sea wall. Many of the guns for that battery are extant, including 8-inch Columbiads, 32-pounder cannon, 8-inch seacoast and garrison howitzers. St. Augustine's Plaza even boasts a converted 32-pounder rifle.