The phrase "small fortress" refers to a small, fortified structure meant for protection and defense. It is a strong and secure place, built to withstand attacks and keep people safe. Full definition
This would not have bothered Runciman, who was not intent on constructing another of the academic «small fortresses that are easy to defend from attack» where «criticism has overcome creation.» (firstthings.com)
Quiet hums softly in my ear; she has a target, and is slowly notifying me of all the weapons, enemies and objectives throughout the small fortress we've infiltrated. (gamingtrend.com)
When a cat climbs inside, the box then becomes a small fortress because it often has only one way in and one way out. (barbaraspetcareservices.com)