Fortezza vs. fortificazione
While both fortezza and fortificazione relate to the concept of a fortress, they are not always interchangeable. Fortezza typically refers to a specific, singular, self-contained defensive structure, whereas fortificazione is a broader term, often referring to the process of fortifying or the collective system of defensive works.
Fortezza
B1This word almost always refers to a specific, large, and self-sufficient fortified building or complex, like a citadel or a castle. It denotes a concrete, single entity. Fortezza can also be used figuratively to mean "fortitude" or "strength of character".
La Fortezza di San Leo è una delle più belle d'Italia.
(The Fortress of San Leo is one of the most beautiful in Italy.)
I soldati si difesero coraggiosamente all'interno della fortezza.
(The soldiers bravely defended themselves inside the fortress.)
Dalla cima della collina, la vecchia fortezza domina l'intera valle.
(From the top of the hill, the old fortress overlooks the entire valley.)
Ha affrontato la difficoltà con grande fortezza d'animo.
(She faced the hardship with great strength of mind.)
Fortificazione
B2This word is more abstract. It can refer to the act or process of building defenses. It can also refer to a defensive structure, but often as a component of a larger system. It is very commonly used in the plural, fortificazioni, to describe a network of defenses like walls, ramparts, and bunkers.
La fortificazione della città richiese molti anni di lavoro.
(The fortification of the city required many years of work.)
Lungo il fiume si possono vedere i resti di un'antica fortificazione romana.
(Along the river, you can see the remains of an ancient Roman fortification.)
Il castello era circondato da un imponente sistema di fortificazioni.
(The castle was surrounded by an impressive system of fortifications.)
L'ingegnere militare era un esperto in opere di fortificazione.
(The military engineer was an expert in works of fortification.)
Summary
In essence, use fortezza when referring to a specific, singular fortress you could point to on a map. Think of it as the final, named building. Use fortificazione when talking about the process of making a place stronger, the general type of work, or the various elements that make up a defensive system (often in the plural, fortificazioni). A fortezza is a type of fortificazione, but fortificazione describes the entire category and process.