Are there ways to gain credits other than donating or buying them at the credit market?
Refer friends to the game!
Oh cool! I hadn't read about referals yet. Awesome. Thanks.
Any other ways?
You can reffer someone to the game, but in order to get your credits they must be active and play, the first time you will get credits is when the player you reffered has reached level 3 
The way the referral system works is your referrals need to reach certain milestones in the game to generate referral points. Once they reach these milestones, you get the points automatically. You can then convert these points to credits.
As long as the referred player is active, you will continue to earn referral points. So one very active player can be worth more than 1000 inactive players. There are already 3 players who have generated over 1000 credits for their referrer in less than a month!
BTW, This is by far and away the best referral system i have seen on any online games I have played