Finished "The Best Defense" quest today.
I played as Justinian I again, had the Great Wall, was the first to have Engineering on my continent and was fighting a war against the Monty and Pacal. I got the task to be the first to have seven Castles before Econimics, Rifling or Railroads.
Reward options
1.) Better training: Free City Garrison I for all melee units
2.) Turn defensive advantage into political capital: +3 relations with all leaders
3.) Now that we are safe focus on learning about enemy plans : +25 espionage points from The Great Wall.
I haven't yet decided between option one and three. City Garrison for melee is nice, especially with the free shock for my Pikemen from a former event but I have muskets now and melee units are soon going to be obsolete. I could build many Pikemen now and upgrade them to Riflemen later, but it's gonna be expensive.
Better relations are quite useless when you are a Confucianists and your only neighbours are jewish Aztecs and Maya.
I have the Great Wall in Constantinople together with two settled Great Spies and I plan to build Scotland Yard soon. I think this will help me most on the long term