Why not just send someone to kill Semiramis and see what happen to her Hanging Garden. I mean, it's like Megaman, where Dr.Wily was defeated and then his fortress suddenly "self-destruct mode" for some reason.
As a matter of fact, Semiramis is a worst assassin servant I've ever seen. Everything she had, the only she uses Presence Concealment during Shirou conversation with Kairi, and the rest she had done in whole volumes was most caster-ish. Well according to history fact, she never associated with assassination action, except she used her magic for her lovers in Armenian legend (Which surprise that Karna notes this) but no mention about poisonous magic or sort of.