This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
A standard Romulan planetary defense system consisted of a network of orbital killer drones that formed an impenetrable shield. Every thirty minutes, a transit window opened in a random spot in the network, giving authorized spacecraft one minute to pass through. Any vessel that approached the drones without clearance was automatically fired upon.
Vashti was protected by a network of orbital killer drones, controlled from Central Station. In 2399, Kar Kantar's Bird-of-Prey drove La Sirena toward the security net, causing the drones to open fire. (PIC: "Absolute Candor")