In the spirit of full-disclosure I will now enumerate the differences that I am aware of between the model and the real thing.
the source of all this trouble; the real thing, courtesy GoogleEarth
The good reader will instantly notice that the actual Castillo de San Marcos is slightly less than exactly square, the seaward face is fractionally shorter than the other three walls. I don't know if this was intentional on the builder's part but I made mine as close to square as I could. One will also see that the parapets on my model as substantially thicker than the ones extant in the picture above. This was done to make them sturdier and allow them to survive contact with wargamers. Also apparent will be the fact that on my model the ravelin is both smaller and closer than it should be; this is a concession to he need to build the model in sections to allow for transportation. Another instantly obvious difference is that my model has a wrap-around moat while the real Castillo replaces the eastern most with a water battery. Lastly my walls are rather thinner than the fortress walls as I was desiring that a player stand inside the fortress and thin wargamers are at a real premium so I had to make the place of arms (that grassy bit in the center) as large as possible.
as I intend on using this model for other purposes
I couldn't resist the urge to add a bit of fancy detail to the front entrance
the real gate is so desperately plain
the actual garetas are thick and short
a simple failure to check drawings left mine far too tall,
this will be corrected as soon as I get the opportunity
the real stairway is a huge arched construction with a very gentle angle of ascent
the need to construct the model in sections meant that my stairway had to fit entirely
in the corner of a bastion, this left it taller and steeper than it should be,
I tried to build a steeper arch but gave up after a couple of tries
the eastern parapet of the Castillo is barely knee-high, this is done to allow the cannon mounted on that face free play over the waters before the fortress, my model has a full-height parapet, again, because I intend on using the model for other games
in a fit of laziness I placed blocks in the corners of the parapets rather than properly mitering the joints, I REALLY did intend on trimming them to match the slope of the parapet but found that I liked the way that they looked and left them as built
then there is the whole matter of the entryway to the ravelin; the real fortress possess a glacis
(a large earthen mound that essentially hides the fortress from observers at ground level)
my model does not have the space for such luxuries
the lack of glacis would leave the entryway to the ravelin uneven, thus I had to build a ramp that roughly matched the slope of the glacis as a stand-alone piece so that the drawbridge would be close to level with the ravelin
I am sure that there are many more differences and will be happy to highlight any that my gentle readers notice and point out in the comments section.