Citadel (noun)

1. A fortress or fortified settlement, typically on high ground, that forms the last line of defence for a city or country.
2. A place of refuge or security.

Origin:

Late 16th century, from french citadelle, from italian cittadella, from città "city".

Examples:

1. The citadel was considered impregnable, but it eventually fell to the enemy.
2. The artist retreated to his citadel of solitude to work on his latest project.
3. In the city, the citadel was a symbol of power and control.
4. The citadel was designed to withstand a long siege.
5. The citadel was used as a prison for political prisoners.