The complex of buildings known as the Tower of London rests on the east end of the city of London, below (that is, downstream and to the east) of London Bridge. It is one of the larger castle complexes in England. Its uses during the Stuart dynasty were military, penal, political, and fiscal.
The center of the Tower is an old, roughly square structure of limestone, whose origins are credited to William the Conqueror. Because of the distinctive color of the limestone, this innermost building is called the White Keep. In theory, the White Keep includes lodgings for the Royal Family and its household and serves as a place from which they might defend themselves from an attack on London; in practice, the White Keep was never used this way and monarch almost never visited the Tower.
The White Keep is surrounded by lawns, which are encased within a ring of walls and towers. Lining the inside walls of the lawn are cottages, which are rented by the Crown to the guards of the Tower and to the support staff who service them. These grounds were the site of many famous incidents, perhaps most notoriously the execution of Anne Boylen under the reign of Henry VIII. Some of those cottages remain in place to this day, and were used to house noble or wealthy prisoners who not only lodged with their jailers, but occasionally visited and dined in style with them as well.
On the outside of this ring of walls, lining the north and east portions of the complex, are the buildings in which the Mint of England is housed. Crucially, the mint complex includes the rooms in which "pigs" or rough cylinders of silver or gold boullion are received, then melted and poured into molds, and re-issued to the bearer. The Master of the Mint (Isaac Newton, for roughly half of the Baroque Cycle) is personally responsible for the integrity of the coins and ensuring that there are no counterfeits in circulation. He is endowed with police powers and military men under his authority to execute these duties, and ensures the integrity of the coins by periodically depositing samples of the coins made under his direction in a triple-locked box called the Pyx. Periodically, a ceremony is held called a Trial of the Pyx, in which the coins are assayed and the performance of the Master of the Mint is assessed. Historically, the Pyx was stored in the White Keep when not in use, along with the Crown Jewels of England.
Another set of walls surrounds the Mint buildings, forming the outer shell of the defenses of the Tower. The outer walls are typically manned with military detachments, and during the phase of history in which the Baroque Cycle is set, the famous Yeoman Guards (also known as "Beefeaters") had not been permanently assigned to the Tower and different military detachments would rotate in or out of Tower duty depending on the needs of those currently in political power.
Prisoners -- both those thought to be guilty of actual crimes and those thought to be politically dangerous to the ruling faction in power -- are housed in the Tower. One of the more interesting things about English prisons in this and preceding eras was the idea that while a prisoner had lost his liberty, there was no partiuclar reason for him to have lost his property, and therefore prisoners were free to have their friends bring in their possessions and money for their use while imprisoned. Those friends, as well as family members and household servants, were free to come and go much as they chose, although there was no pretense of privacy from the guards and thus from the political authorities. However, for the same reason, the Crown had no legal duty to provide a prisoner with more than minimal sustenance and impoverished prisoners often had miserable existences. Commonly, prisoners were brought in through a boat dock on the south side of the Tower, which abuts the Thames River, called the "Traitor's Gate."
Spoiler Warning: Various characters in the Baroque Cycle are imprisoned in the Tower at different points in the story, including Daniel Waterhouse. They enjoy the accomodations (or not) in variance with their personal wealth, the willingness of their free friends to assist them, and their ability to form friendships with Tower Guards. Attempts to escape the Tower are part of England's romantic and heroic history, and there is much discussion of escape. Jack Shaftoe, however, masterminds an elaborate and spectacular attempt to break in to the Tower.