Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
The fortress in a dream is religion and knowledge that fortifies its owner. Entering it indicates adherence to religion and salvation from strife.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
The fortress indicates an invincible man or a ruler. It is security from fear. Whoever sees himself building a fortress is fortifying his religion or his world.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
The fortress is interpreted as the king or the scholar. Entering it indicates marriage to a fortified virgin woman.