In a metaphorical sense; Literally,'Befestigungssperre' is " a fortified/ fortification barrier

Austro Hungarian barricade. Befestigung is fortification here but given the sperre and rest of the context, I would just use barrier.

See reference.

Compare with this:

"Generally, in Austrian military terminology these defensive structures were not referred to as forts. The terms used to describe them were ‘werk’ (’works’), ‘sperre’ (‘barrier’) or ‘straßensperre’ (‘roadblock’). Defence districts were referred to as ‘rayons’. Originally there were only two, divided into subrayons or sections. Over the course of the First World War, however, more and more subrayons and sections were replaced by rayons."
https://en.wikipedia.org/wiki/Austro-Hungarian_fortification...