How did Emma and Jacob try to cross a gap in the bridge to the fortress?

Emma and Jacob used the pikes that had held heads to form a metal bridge to close the gap.

We ran back and forth, prying up pikes and laying them across the gap. The heads spat and swore and issued empty threats. When the last of them had been pried off and rolled away, we’d made a small metal bridge, roughly a foot wide, slippery with head goo and rattling in the ashy breeze.