Metal Slug Defense is not entirely hot garbage, but there is better to be found in the realm of defense games where you get resources constantly and build units to push towards an enemy base that you destroy. Two things that annoyed me right up top? Full-screen ads on the mission select screen and full-screen ads right after you've completed a mission. No matter how fun or charming a game can be, having a video ad for some other game block your device is awful.
Full screen ads aside, this game does it all, energy systems, premium currencies, best value 99.99$ bundles and one-use items you can buy with premium currency that completely ruin any form of balance there could ever be in a game. But otherwise, it's free, and there's plenty of content you can theoretically go through in it. If you have metal slug nostalgia, you might remember fondly the characters they present to you, but otherwise, it's just pretty neat pixel art without meaning (that being said, even I, someone who never touched Metal Slug, recognize the RAWKET LAWNCHAIR voice clip)
The way the game works is simple, you have a deck of units that cost AP and have various stats and attacks/abilities (that you use by tapping them whenever their gauge fills up), you have a little base and there is an enemy base on the other side of the screen. Your AP fills up over time and then you can use it to buy units - which them triggers a cooldown before you can buy them again - or upgrade your AP production to make more AP to build units faster. There is a wide range of units with many special abilities but you won't see most of them since the only way to get them is to buy them with premium currencies. During battles you will also find hidden resources and POWs.
The cash resources are used to upgrade your base - its hit points, how fast you get AP, your maximum AP, etc. - I believe them if they say the upgrades do anything but I wish it shown my actual AP production rate and how much I would increase it by upgrading - you can also upgrade your units up to level 10. POWs upgrade your units in different ways if you collect all of them in each world - again, you don't see by how much - but I hate the actual POW collecting since they are invisibly hidden in each map and the only consistent way to find them is to randomly use arcing weapons because their spots ten to be in mid-air. Since they're random, it takes many run of the same levels to get them, but you have to flail around just to get a chance to uncover them. I would have rather received them randomly for completing missions.
To finish, speaking of missions, I find the balance laughable and almost irrelevant to the grand scheme of this game. My strategy for all the levels has been to only make the smallest cheapest unit non stop forever, the little soldier guys with shields, guns, mortars and grenades were the only things I ever built and it worked against tanks, giant boss crabs and other enemies. No matter if the level said "hard" or "easy" or "very hard", I would just level the AP production, then build an endless stream of little soldiers, and never even got my bases scratched. A bit weird, especially with all the units they're selling for quasi-real money. If you can already beat most of the game just with basic soldiers, what's the point?
MSD isn't a terrible game, but it's not very good, they get way too aggressive with the advertisements and other F2P annoyances, but at least it's charming and there's plenty of stuff to see in it.