Most phones and tablets are very limited on RAM memory. We recommend 1.5 GB of RAM, but some devices have only 1 GB, which means memory is very tight. That's still enough in most cases -- but on some devices, the particular RAM usage of the device means that it does run out of memory at these moments.
If the game crashes on you, try doing this:
Shut off the device, wait a minute, then start it up and launch the game. That will make all available RAM available to the game (no other apps will be using any RAM).
In 'Game Settings', choose the 'Fast' quality level.
Switch to Day (time of day) before you finish marking your territory. You can do this by tapping the 'Rest' button (laying down wolf icon), then tapping it again after it changes to ZZZ button, then choosing Afternoon and Sleeping.
Those should reduce or eliminate crashes on any device that can run the game.