《北齐书·本纪·卷二》
于是堆起土山，挖掘十条地道，同时又在东边开挖了二十一条地道作为攻城的手段。
乃起土山，凿十道，又于东面凿二十一道以攻之。
<translation>So he constructed a mound of earth and excavated ten underground tunnels, while simultaneously excavating twenty-one tunnels in the east to serve as means of attacking the city.</translation>