Spy missions can no longer be performed in cities of a player you have an Alliance with. This is incredibly disappointing and frustrating. As @acluewithout said there is plenty of history of allies spying on each other, and currently this is one of the few subtle endgame mini-games that lets you thwart others from winning without having to nuke them (literally) or nuke a long-standing friendship/alliance by declaring (non-nuclear) war for the sake of distracting them from victory. I would much prefer they institute a harsher penalties for being caught spying on your allies - something like you gain no further alliance points until it is up for renewal (also pleeeease let us renew alliances before they expire) or even get dropped down a tier. Plus a harsher relationship penalty from the aggrieved party.