In terms of structures that do or don't survive the fire, you might look into the Great Fire of London (mid 17th century) as an example. Large sections of London survived more or less unscathed, while other sections were practically burned to the ground. The same has been true of most whole-city fires (Chicago, San Francisco, Seattle).
Wholesale destruction, however, is the assumption, when wood frame structures are in direct contact or separated by only narrow streets over a wide area (typical medieval/renaissance construction). Once the fire is burning well, embers may jump even a broad avenue (or a river) and start the fire anew on the other side, especially if roofs are flammable (wood shingles or thatch, as opposed to tile, for instance). On the other hand, an area that's upwind of a burning area may be spared even in absence of effective means to fight the fire(s).
As far as simulating the fire, I'd start with knowing the wind direction. The fire will generally tend to move downwind, though if it gets large enough it may begin to create its own "weather" -- mainly inflowing winds from all directions leading to a firestorm similar to the ones that consumed Dresden and Nagasaki during the Second World War, though there have been documented cases of large forest fires generating their own rainstorms (this requires that the air drawn in be humid enough to precipitate when it rises sharply).
For fires that start on the outside of a building, it's generally structural damage that destroys the building -- the roof will burn and fall in, or the walls will collapse, both before the interior is gutted, in direct contrast with a fire that starts inside, and may completely gut the interior before the walls or roof come down.