Entangle says that if you make your save, but are still in the area at the end of the caster's turn, you have to make another save.
What if you failed the save and are still in the area?
It might sound silly, but in the case of an area with thorns, you take damage on a failed save. This would indicate that you either make a save every round to avoid damage, or you have the option of not bothering to escape to avoid damage.
If you do have to make your save each round (regardless of if you are already entangled), does making that save just avoid the damage, or will it break the entangle effect and set you free?