Objective: study the "Vision" science, build a Visionary Tower, train Visionaries, observe enemy's missions
Hint:
In order to repell the Visionaries you can build the Mirror Towers where magic Mirrors that reflect wizardry are produced. To get the information on enemy's missions there has to be more Visionaries than Mirrors. Each Mirror has a high chance to kill a Visionary, but a Visionary has only a slight chance to kill a Mirror. If Mirror Tower is damaged or destroyed in the course of the "Buildings Destruction" mission all Mirrors in the Tower disappear. If the city Gate is closed, Mirror Towers cannot reflect Visions.