|Here is an image of how the prison looked back in the day.|
A bit ironically, they have erected his statue there to commomorate him, or his prison spell here...I've passed it by about a million time but never really paid attention to it.
So, what was he doing in the prison? It appears that he was working as a tax collector in Andalucia but got his books cooked up a bit. So he had to pay his debt to the Crown, which meant that he had to collect more money. So he did, but unfortunately to him, the bank where he deposited the money got bankrupted and he lost it all. That ment that he had to serve his time in prison.
Apparently, while there, he was helping his fellow illiterate prisoners by writing letters on their behalf! Also around this time, he started thinking about Don Quijotte as a character.
This travel site talks about Cervantes' Seville in English. There are a few things in town, like this walking tour with tiles around the city (see below). This page has more about him and Seville (in Spanish).
Miguel de Cervantes Saavedra (Alcalá de Henares, 29 de septiembre de 1547 - † Madrid, 22 de abril de 1616), llamado el «príncipe de los ingenios», escritor.