You can click on the thumbnails for a larger view. However, I have to fix a few of the photos. I am not sure why you get a screen saying "image unavailable" when you click on a few of the thumbnails. Hope to improve this pronto. I got fed up with dealing with the Typepad editing abilities so I just stopped.
San Juan de Ulúa is a five hundred year old accretion of pre-Hispanic, Spanish and Mexican history that sits on a semi-detached island across the water from downtown Veracruz. Today it hides itself in a strung-out living monument to globalization and NAFTA, a giant erector set of derricks and almost fanciful modern harbor constructions, and a port full of ugly container ships. One gets to San Juan de Ulúa through the new customs area. passing hundreds and hundreds of giant boxes and drums unloaded from ships and stacked and waiting to be loaded on eighteen wheelers which sit like little subdivision houses in orderly rows of pull-in parking spaces. A nest of brand-new customs buildings is in process of construction with few windows and little attention to anything but functionality. Everything is brushed with sand.
Around a curve in the road suddenly there is life: a strip lined with and souvenir vendors and tourists unloading themselves from buses and cars and men waving rags trying to get you to get to pay to park. The walk to San Juan de Ulúa is thus rather festive.
Originally a guard post, now the admissions area, this building shares the turf with some very modern and very large port equipment. Fortunately, not all the trees have been knocked down. Perhaps you can imagine when it was sandy shore and empty island and well-dressed Indígenas lined it to see the strange flotilla approaching.
San Juan Ulúa's history is woven inextricably into the history of Mexico: Indígenas' shrine, place of disembarkation for conquistadors, port, fort, prison, would-be presidential palace, military headquarters. Today, it functions to keep alive its corner of the Mexican past. It's called a museum, but aside from a tiny display area, there are no labels, no glass cases, no ropes on poles to keep the too-curious at a distance, no well-marked routes, no railings to keep you from jumping into the sea. You are free to wander unnoticed by guards, unwatched, under arches, up stairs, through the cavernous rooms, up on the roof and out into the sun-bleached courtyards. Everywhere you see one century piled on another, can imagine you hear echoes.
Here is a link to a pretty good diagram of San Juan de Ulúa so you can identify features as I describe them.The highlighted text Here is a link
The most common building materials for the walls were embedded coral fossils. Also used were rocks of calcium carbonate and marble and bricks and wood. There's a beautifully illustrated document by Jose Hernandez Tellez detailing the construction and its history and proposing means for restoration which has been undertaken. the document is at this link.
These blocks were stacked at the entrance the day we were there. I think they are to be used in restoration efforts. I also believe they are made to be the same as the original blocks were.
The pages and pages and pages of Spanish and Mexican history tend to obscure our view, but the islota, the little island which became San Juan de Ulúa was known to the indígenaswell before any European saw it and in fact, that first Spaniard, Juan de Grijalva, was reported to have arrived on it to find four Totonac priests taking part in a ceremony devoted to Tezcatlipoca, a creator god associated with darkness in the shared Nahua pantheon of the place and the era. The area of Veracruz itself was called Chalchiuhcuecan. William Prescott's description of Cortés approaching and landing on San Juan de Ulúa and then meeting with the people who lived there can be found here. Download william_prescott_on_corts_landing_at_san_juan_de_ulua.doc
With the initial arrival of the Spanish, San Juan de Ulúa became the accustomed anchoring place for boats and ships because it was both deep enough and somewhat sheltered from the wind, though as Jesus Hernandez Tellez makes clear, not enough to prevent deep scarring and structural damage over the centuries. Hernán Cortés spurred the development of San Juan de Alúa as the principal harbor for sea traffic to Veracruz. He moved La Villa Rica de la Vera Cruz three times: from its site across from San Juan de Alúa to an area up the coast just east of Quiahuitzlan on the coast (see a future posting), then to La Antigua and then back to the present site of the city of Veracruz, but the port remained at San Juan de Ulúa. All goods and people had to pass through it. In 1542, the first dock was constructed. By 1584, it had an artillery tower, houses, a church and a hospital.
The wall surrounding the door through which you can see some of the port probably existed in 1584. It is the South Wall. I am translating cortina as wall. A cortina connects two fortifications called baluartes. The cortinas on San Juan de Ulúa contained rooms of various kinds.
As the wealth pouring through the port increased, and as Spain engaged with other European empire builders, the need to defend against pirates and naval attacks grew. Among the earliest attackers of note to USAers were Sir John Hawkins and Sir Francis Drake who arrived in 1569. The Spanish largely destroyed their flotilla, killed most of the sailors, and drove the few survivors away. From the Spanish perspective, they were indeed ruthless pirates deserving of their fate. Sir Francis Drake proved his perfidy by surviving to torment Spanish ships again and again.
But, wait, this is the Spanish version. The British saw this incident at San Juan de Ulúa quite differently. According to the history on the site of the British Royal Navy, Hawkins and Drake were captaining no more than a small "Squadron of six ships" and because of storms "were unable to put into port until San Juan de Ulúa was in reach for their safety." They may not have come as friends, but they certainly weren't enemies. It's not a long story, and you can see the British version here: here
It was in the 17th and 18th century that it took on the character of a fort. The engineer Jaime Francke undertook the project. The viceroy, the Count of Monclova, approved it.
Below you see the courtyard and El Baluarte de San Pedro. The plantings are contemporary. 
Here is El Baluarte de San Crispin with its Torre de San Felipe.
These two Baluartes anchor the South Cortina.
Here you can look through archways stretching under The North Cortina.
And a stairwell
And a view of the roof 
And some glimpses of La Plaza de Armas, or the main plaza.
There were fortifications built away from the main islota. The first photo is of El Revellín de San Jose including a construction called La Media Luna and the second of El Baluarte de Nuestra Selora del Pilar. 
Throughout much of its history as a fort San Juan de Ulúa also served as a prison. At times captives were reportedly chained to the outside walls and left to the mercy of sun and heat and hurricanes.
Probably the walls appear here as they did to the prisoners, but not much more does.
I don't know, aside from the outside walls, where the prisoners were housed, but their cells might have looked like this:
Among the prisoners were fray Melchor de Talamantes and fray Servando Teresa de Mier arrested for anti-Spanish political activies in the early 19th century. Mier, in fact, seems to have been held prisoner at San Juan de Ulúa in the dying days of Spanish domination. For political activities following independence, Melchor Ocampo was imprisoned there. During Porfirio Díaz's era, apparently thousands of political prisoners were jailed there.
The Mexican Revolution, from 1910 to 1921 (more or less) led to the overthrow of Porfírio Díaz, a man who brought dreams of modernization to fruition but destroyed dreams of individual freedom and the rescue from poverty of the poor. I have always thought that USAers would do well to study the Porfirian period to see what happens when progressivism runs amok. He became especially adept at anesthetizing the upper and upper middle classes. One of those who rose and fell during The Revolution was Victoriano Huerta who played to the rich in Mexico, Britain and the US, notably by protecting and increasing their oil interests.
During Huerta's rule, the US intervened directly in Mexico with a massive and deadly attack on Veracruz, finally leading to occupation. Needless to say, it did not make thecountry popular among the masses of Mexico.
In any event, eventually, Venustiano Carranza, a Constitutionalist from the State of Coahuila, became head of the military and then President. It was his desire to demilitarize San Juan de Ulúa and then move the President's residence there. The photo below is of what is called The Governor's House. I believe this is what was constructed at Carranza's instructions. I don't think a Mexican president ever lived in it.
San Juan de Ulúa was besieged at least two other times by the US during US invasions, the first time during the Mexican American War of 1846 to 1848. The US invaded Mexico for a second time during the Mexican Revolution in 1918. It is incumbent upon us to learn more of how our history is interwoven with Mexican history.
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A couple of current notes:
While San Juan Ulúa did receive international as well as national funds for its restoration, it was denied declaration as a Cultural Heritage of Humanity by UNESCO because of "the visual contamination of the industrial and port zones which surround it."
The very popular opera singer Andrea Bocelli was to sing in San Juan de Ulúa in 1998. I don't know if he did.