The magical shield over the crawler base, proof against dropped bombs but bypassed by paratroopers.
The shield around the base was raised by a group of human mages — including Reid Gunn — working with the former crawlers. Its existence is exactly why Carl concludes the destruction of the Home Base Anti-Air Battery and Castle Hangar Bay must be sabotage rather than bombing: the shields would protect against a dropped bomb, so the Monkeywrenchers must have been inside.
What it does not stop is troops. Tipid reports that the incoming craft are transports, not bombers, and that Paratroopers are getting through the protections — a gap that, together with The Reavers' saferoom door and the Brain Worms, leaves the base breached from three directions at once.
Base-wide magical protection that stops ordnance but not infiltration.