Sifrei Devarim 204

¹(Devarim 20:20) "Only a tree that you know": This is a fruitful tree. ( Ibid . 19) "to come before you into the siege": Cut it down. ²( Ibid . 20) "that it is not a fruitful tree": This is a barren tree. If in the end we include (to be cut down for the siege even) a fruitful tree, what is the intent of "that it is not a fruitful tree"? We are hereby taught that a barren tree takes precedence (to being cut down) to a fruitful tree (if only one is needed for the siege). I might think that this holds true even if the barren tree is worth more, as per the words of R. Elazar b. R. Shimon; it is, therefore, written "Only" (to exclude such an instance). ³"and you shall cut": Make of it barriers and siegeworks. ⁴"and build the siege": Make siege engines for it. ⁵"against the city": Bring up catapults against it. ⁶"until it is subdued": even on the Sabbath. This is one of the things expounded by Shammai the elder.