The watchtower line of buildings gives pretty good happiness bonuses at very cheap prices. First things to build usually. Church is next and then monastery and cathedral. You can find all the bonuses by investigating the build prod file in the appropriate column.

Chrstian factions do not raid Christian buildings, even if they are of a different Church. Thus Catholics leave Orthodox churches, monasteries, reliquaries and cathedrals in place and use them as theirs and vice versa. Different religion factions do raid all religious buildings though.

Spies do give a bonus but not a cumulative one - ie their effect doesn't stack unlike in STW. Instead the spy with the greatest valor is counted of the lot.