The impression I always got was that the shield was like a flat surface above the base; it protected the base from an aerial bombardment but it was possible to sneak under the edges at near-ground-level (ie. with walkers or speeders).
The impression I always got was that the shield was like a flat surface above the base; it protected the base from an aerial bombardment but it was possible to sneak under the edges at near-ground-level (ie. with walkers or speeders).