What to do when fiends close to the gate?

As you can see when you have fiends close to the gate and you do not enough hearts to save yourself so there are more than one solution and to know the solution would be in the second attempt when you are to enter the new level and to try it again.

Because if you start the game again with the same formation and the same towers you could see this scene again so whatever you have to do is to do some changes that I would to take to you about as first one is to use a spell.

Using Venus Fiendtrap would be a good option to put it at the gate and it will get rid of 5 fiends before they could enter the gate and second spell but it is less effective which is Raining cats that also could damage a group of fiends if their health is low.

You can use a strong upgraded tower just close to the gate and it will do the mission for fiends that have escaped from towers and about to enter the gate but if these fiends are bloodlings like below so you have to get a distance from the gate and have a tower with good range.