Yamps
Deity
I can only influence success percentage by targeting capital/non-capital cities and stack counterspies by the defender.
Really? There shouldn't be a difference from the capital nor from stacked defending spies. Bhruic explicitly stated that for stacked defending spies.
http://forums.civfanatics.com/showpost.php?p=6063324&postcount=11
OTOH, according to
http://forums.civfanatics.com/showpost.php?p=6076611&postcount=30
the reduced success from stacked attacking spies shouldn't happen either. But it does, I could give similar screenshots like Dirk. I remember that the last spies attacking Mansa had higher success chances.