Greetings, Z4 Fighters!!
About Zombie Defense, we've got many complaints about Zen rewards since after the last maintenance.
However this was a kind of 'bug' which was not intended to be, so it was fixed considering Zen balance in the game.
But also we understand it might hard to get enough Zen from game playing so we discussed with our team about that.
There will be rising Zen rewards on clearing 20 rounds soon upcoming updates. =)
Please don't hesitate to give us your feedback. We are here to listen.
Thank you in advance!
~Zone4 team