Its value in war was such that only after it was built did thebrotherhood decide to start the siege of Dracula's castle.
从句部分倒装，built 和 did 之间无逗号，请老师剖析一下句子。
这是因为only在句首引起的部分倒装。假如不倒装便是：The brotherhood decided to start the siege of Dracula's castle only after it was built.
