I think two changes will increase the number of active players significantly.
First, when someone loses all castles, he should be given option whether he wants to restart with a new castle, or just view the map, stats, etc. And if he chooses to restart, maybe he can have the option to start in a free castle of his choice? That will reduce the number of 13 point castles.
Second, reduce the time for everything by half. Travel time, build time, and production time ( that means double the amount of resources produced). By doing so, it takes 1 day to save silver for a castle, which keeps players active every day. The game gets boring because some days there is nothing to do.