Hello Well done on this game so far. things I noticed, on the initial screen Yeti home defense logo could be much nicer maybe some embossing and a Photoshop overhaul. If you don't have or cant afford Photoshop you can use the online one similar to Photoshop called pixlr its free.

Your main menu could use some touching up as well, its a bit flat, and that text could use some life as well.

The next menu where you pick the difficulty is much much better, But I am assuming that was what came with the Unity asset package. However you could change the color a bit and easily use those on your main menu as well.

Once in game we need a way back to the main menu or back to the difficulty screen. I would put a small menu to go where we want somewhere on the tower upgrade area. maybe a small one in the top left corner.

You can also create a button to exit the game as I can see you have nothing recognizing the escape (back) button on the device. You can do this easily by creating a button in Unity. Add the button to the canvas anywhere, change the text to whatever you want (Close game, Quit, or just an X) Create a C# script and name it whatever you wish Such as (CloseGame or QuitGame) this the only script you will need to just exit the game.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CloseGame : MonoBehaviour{

public void quit() {
Debug.Log("has quit game");
Application.Quit();
}
}

Save that script in its own folder in your Scripts main folder. Drag that script to your button in the Hierarchy. Then In the button script (click on button in the Hierarchy then open the button script in the inspector) click the + on the onclick and drag the button from the Hierarchy to the on click area under runtime and choose CloseGame (or whatever you named your script) as the function.

The gameplay is nice and progresses well, however the impossible difficulty could be a little harder. Other than that it is a very good first game. This is one of those hyper games you can change and change and change and publish over and over just by changing the themes and object graphics inside the game. Good choice for a first game.