The name of a strong building for defense is a fortress.
Other names of a strong building for defense include fort, or even fortification.
These terms are terms that refer to areas or structures that are fortified to withstand attack and are also often associated with military or strategic importance.
Forts are often smaller defensive structures than fortresses.
Fortresses are often larger and more complex defensive structures and buildings and often contain troops and other resources.
Fortifications can also be man made structures such as walls or natural features such as hills which are used to strengthen a defensive position.
A fort is basically a strong building or a place with a wall or fence around it where soldiers can remain and be safe from their enemies.
There are also other names used to describe strong and defensive buildings which include bastion, stronghold and citadel.
Although these terms also sometimes have more specific meanings.
A fortification is a military construction designed for the defense of territories in warfare, and is used to establish rule in a region during peacetime.