You're completely right about the defense. Right now you can only get lvl 1 mercenaries, these are too weak to defend against most enemies. I'm working on a feature that will allow you to train your troops to a higher level for money. It will be ready sometime next week.
With respect to analytics, I collect some event data, but not enough to draw solid conclusions. I'm having trouble figuring out what type of metrics to collects that would be helpful. I have win/loss metrics for the player in general, for each unit specifically, talent learning metrics, item usage metrics, button clicks metrics. But it's hard to draw conclusions as the metrics engine (Google Analytics) doesn't let you transform the data (for free that is). I have learned a bit from these metrics, but not a huge amount. Do you have some ideas?