This will probably be outrageously simple when someone who has read the novel explains it, but the ROCH '83 series and the comics adaptations leave it nebulous.
At the end of RETURN OF THE CONDOR HEROES, the Golden Wheel Monk had Gwok Seung tied up on a tower outside of Seung Yeung Fortress. So well-guarded was the tower by the Mongol army that even the power of four Greats (Gwok Jing, Chow Bak Tung, Wong Yerk See, 1 Deng), numerous first-class fighters (Wong Yung, Yeh Lut Chai, Ying Goo, Ching Ying, the disciples of 1 Deng, etc.), and a large force of Sung soldiers, beggars, and Cheun Jen Sect Taoists using Wong Yerk See's specially-designed battle formation could not fight their way through the Mongol defenses to rescue Gwok Seung. Yeung Gor, however, was able to swoop in and pull off the rescue. How was he able to approach the tower so easily?
In the TVB adaptation, it was simple: the Divine Condor could fly, and Yeung Gor and Little Dragon Girl were riding on its back. Hence, Yeung Gor was able to skydive onto the tower, bypassing the Mongol army. In the novel, however, the Divine Condor *cannot* fly. Did Yeung Gor use the Ancient Tomb Sect's superior hing gung to approach the tower? Did he strike from the back (all the Mongols' resources were focused on stopping Gwok Jing, etc., in the front)?