One of my fears with mobile free to play games is how annoying the gameplay will be to get you to pay money. I don't fault developers for doing what they need to do to pay rent, that said, I prefer buying games up front. That said, that's not how things often work on phones so free to play it is and Mighty DOOM is nice in that it doesn't seem like you need to pay for anything. Well, I could see a time where advancement might be so slow that paying makes sense, but I imagine i'll stop playing before that becomes a problem.
Mighty DOOM is a top down bullet hell shooter where you play through each stage one level at a time. I'd say Mighty DOOM feels more like a space shooter like Galaga than the traditional DOOM first person gameplay. That said, the bullet hell styled game play is on point and there is quite a bit of strategy you can employ in each stage to keep the gameplay fresh. There is a progression system by which you collect coins to upgrade your weapons and armor. Some weapons and armor have different abilities that can change your gameplay well worth experimenting with. For instance, if you want to stand back and shoot things from afar you should level up the rifle but if you want to get in close for more close quarters DOOM gameplay, you need to upgrade the shotgun.
So far, I've barely scratched the surface of Mighty DOOM and it's already a lot of fun. If past Bethesda mobile games like Fallout Shelter or Elder Scrolls: Blades are an example, Mighty DOOM should have a long life slaying demons ahead of it.

Sounds great, thanks for the review! I like your Galaga comparison. I'll have to keep this in mind when ever I think of gaming on my phone again. It's not something I usually do as I'm used to having phones that aren't great. I've thought about buying a new phone, but I'm not too eager or enthusiastic.