'Strong bread' is a rich, dense, long lasting fruit cake is said to have been used as currency on occasion in the Middle Ages, and added a note of cheer to the provisions of the Crusaders. Its 'strength' is derived from the black pepper (hence its alternative name in other parts of central Italy, panpepato).
Siena is the capital of panforte, and you'll see it year round in Sienese shops. Traditionally it has 17 ingredients, as many Siena has contrade, or neighbourhoods, who famously compete in the city's Palio:
candied orange peel
brewed espresso coffee
red currant jelly
Images by: oetker