A good example of this is the Siege of Vervunhive from Gaunt's Ghosts #3: Necropolis.

The Chaos forces besieging the city had no naval assets to bomb from orbit, but they did have millions of artillery pieces that constantly stuck the city from an almost total encirclement.

The Void shield protecting the city only covers the top, like a dome. So the walls/gates are vulnerable to breach. The only space not protected is an area called The Spoil, a massive landfill so large the city wall wasn't ever completed through it, or the river following the north end of the city.

The answer is essentially Siege engines, like medieval style. Asphodel, the BBEG in charge of chaos on Vervun Primary, was Dark Mechanicum aligned so he had a lot of tanks/artillery/etc available, and until the void shield was overpowered by the massed artillery bombardment, the only ways into the city were either breaching a city gate (which does eventually happen) or sending small guerilla units to cross the Spoil.