TT #329 · Parashas Shoftim · Seventh Aliyah · Passuk 20:20
Only a tree that you know is not a food tree – that one you may destroy and cut down, and you may build a siege-work against the city that is making war with you, until it falls.
Torah Temimah points to Sifrei Devarim 203.
TT lemma: וּבָנִיתָ מָצוֹר.
English: ‘And you may build a siege-work against the city’ – from the non-food trees.
Derash limiting the building material (Sifrei) — siege construction uses only the permitted trees.
Siege works may be built from non-fruit trees; the fruit trees remain protected even while the city is under attack.