Era

The game has 5 chapters, called Era. Each chapters has a specific enemies pool and boss. Also each Era has a time line of 7 Days (each one with a pool of missions) and progressing to the next days refresh both the missions and your daughters active status (each daughter can fight only once per day).

When the player reach the 4th day of any Era, its Boss is discovered and can be fought (anyway is better to complete all the missions in all the remaining days to boost your daughters and collect more resources).