Fortaleza vs. fortificación vs. alcázar
While fortaleza, fortificación, and alcázar can all be translated as fortress, they carry distinct nuances in Spanish. Fortaleza is a general term for a large defensive structure, fortificación refers to defensive works or the act of fortifying, and alcázar denotes a specific type of historic Moorish fortress-palace.
Fortaleza
B1This is the most general and common term for a fortress or stronghold. It refers to a large, heavily fortified building or complex, designed for defense. It emphasizes the inherent strength of the structure. Fortaleza can also mean strength or fortitude in an abstract sense.
La vieja fortaleza dominaba el valle desde la cima de la colina.
(The old fortress dominated the valley from the top of the hill.)
Se necesitaron décadas para construir semejante fortaleza.
(It took decades to build such a fortress.)
El castillo fue una fortaleza inexpugnable durante siglos.
(The castle was an impregnable fortress for centuries.)
Admiro su fortaleza para superar los momentos difíciles.
(I admire her fortitude to overcome difficult times.)
Fortificación
B2This word is more technical and specific than fortaleza. A fortificación is a defensive structure or a system of defensive works like walls, towers, bastions, or trenches. It can refer to a single defensive element or the entire network. It focuses on the result of the act of fortifying (fortificar).
Las murallas son la principal fortificación de la ciudad medieval.
(The walls are the main fortification of the medieval city.)
El ejército construyó una fortificación temporal con sacos de arena.
(The army built a temporary fortification with sandbags.)
La fortificación de la frontera fue una prioridad para el general.
(The fortification of the border was a priority for the general.)
Toda la costa estaba protegida por una serie de fortificaciones.
(The entire coast was protected by a series of fortifications.)
Alcázar
B1This is a very specific term with a a clear historical and architectural context. An alcázar (from Arabic al-qaṣr, meaning the castle) is a type of fortified palace in Spain, built during the Moorish rule. It uniquely combines the functions of a defensive fortress and a royal or noble residence.
El Alcázar de Segovia parece un castillo de cuento de hadas.
(The Alcázar of Segovia looks like a fairytale castle.)
Visitamos los hermosos patios y jardines del Real Alcázar de Sevilla.
(We visited the beautiful patios and gardens of the Royal Alcázar of Seville.)
El rey vivía en el alcázar, protegido por sus altas murallas.
(The king lived in the alcázar, protected by its high walls.)
Muchas ciudades de Andalucía conservan restos de su antiguo alcázar.
(Many cities in Andalusia preserve remains of their old alcázar.)
Summary
In summary, use fortaleza for a general, large stronghold, much like the English fortress. Use fortificación for specific defensive elements or systems, similar to fortification. Reserve alcázar exclusively for the historic Moorish fortress-palaces found in Spain.