Short answers, pulled from the story.
An insula was a multi-storey apartment building that housed most of Rome's urban population, including plebeians and the middle classes. The ground floor typically contained shops called tabernae, with residential apartments on the floors above. Owners were often wealthy Romans such as senators.
Insulae could originally rise as high as nine storeys. Augustus capped their height at about 70 Roman feet (20.7 metres). Emperor Trajan later reduced the limit to 60 Roman feet (17.75 metres), and Emperor Nero imposed additional restrictions after the Great Fire of Rome in 64 AD.
The 4th-century Regionary catalogues record between 42,000 and 46,000 insulae in the city during the late 3rd century, compared to roughly 1,790 domus. These figures are used by classical demographers to estimate Rome's population.
The Insula dell'Ara Coeli is the only surviving insula in Rome. It is a five-storey structure dating from the 2nd century AD, located at the foot of the Capitoline Hill.
Upper floors were the cheapest and least desirable because they were the furthest from safety in a fire or collapse, and required climbing extra flights of stairs. The smallest apartments were placed on the highest floors, while the largest and most expensive units occupied the lower residential levels.
Luxury insulae at Ostia Antica featured a central rectangular room called a medianum from which all other rooms were accessed, large glazed windows overlooking gardens or courtyards, ornate exterior columns, and upper floors with kitchens, latrines, and piped water. Their decoration suggests wealthy, long-term residents.