Stable

Stables were sheds in Rome that the Assassin Ezio Auditore da Firenze could renovate to increase the availability of horses and Rome's income. Once the first stable was renovated, Ezio could whistle for a horse anywhere in the city and one would come.

History
Before the Liberation of Rome, the city's stables were in disrepair and disuse, until Ezio Auditore came along and renovated them, increasing the amount of available horses in Rome and the amount of money Ezio received in banks.

Reference

 * Assassin's Creed: Brotherhood