Siege Tower is a Offensive tool that can be bought whith 12 rubies per tool when you have Siege workshop level 2. It decrease 15% of the enemy wall per tool. Its travel speed is 70. You can earn the tool also by collecting then in some nobillity contents, the Wheel of Fortune, and also at the mission posts. With its low cost, you could buy this tool if you have the "Build tools" Daily task to do, while also doing the spend rubies task as well.