Version 0.6 includes the following updates
Windowed Mode
This feature was requested by a user on the discussions page. I was in the middle of working on the background graphics and art but I found a nice place to pause and squeeze this feature in.
Background Art + Graphics
I think background art requires patience to get right. The background is constantly seen by the player, so in order to be pleasing it needs to be subtle. At the moment I am constantly implementing and removing new ideas and trying to select the aesthetic that feels right. There is no formula for this, so it will just take time.
What's Next?
Version 0.7 will include the addition of the final card. Let's just say we can't have a medieval themed tower defense game without flags :)