What's the fastest way to get high level spies? I've usually put my spies into my own capital (the first spy - because hey, spies are immortal, therefore KNOWLEDGE and WORK EXPERIENCE in espionage is priceless!) High level spies are crucial sometimes in late-game for couping enemy CS, for diplo win. Does it benefit your levelling-up of spies, if you build constabulary line of buildings? Or what exactly does the building line do? Does it simply make it slower for AI spies to steal tech?