I would not say I am a tower defense game guru, but I can hold my own generally unless the game expects me to somehow figure out a tactic no one ever hinted at me before. I can perform a deep dive when I had to, and I did so for the game Defense Grid.
If I play in easy mode, I don't expect to breeze through EVERY mission, but I don't expect to repeat the third mission multiple times and getting no hint on why I am not succeeding. This points to a problem in game design... bad difficulty curve, lack of hint system, or expecting the player to read the minds of the mission makers. And as it turned out, it was one of two major flaws in MarZ.
So I decided to look for some help. After watching someone who aced the mission (no casualties, no damage to lander), he did it by fighting one or two fronts at a time, NOT mining resources if not needed. In other words, he's very quick in selling off his towers (or even refinery) to concentrate on a different front. You need the 100% refund perk to make this work though. Normal sell price is a 20% loss, and you need to continue mining to make up for that.
This is where I also noticed I did NOT pick my perks... or used my slo-mo, and pause button. But really, how was I supposed to notice? If they want us to pick a perk, they should just tell us. And also with the "sell everything and rebuild it in a different direction". This could have been easily fixed by adding a few bits of scripted dialog between the levels, like "they are coming from the northeast!" and "Commander, sell off the towers and rebuild them to the northeast!" and so on.
You can even make it reactive. If you hadn't rebuild the towers, make the dialog go "Commander! Build the towers toward the northeast! Hurry! Sell off the existing towers if you have to! You have the perk! Use it!" Though I guess the perk doesn't fit into "reality".
Which brings us to the other problem of MarZ... lack of alternate strategies and/or tactics.
Most tower defenses ended up being "puzzles" instead of wargames... Albeit with very flexible solutions. You can often achieve the same results with a variety of mix of towers, depending on your specific tower defense genre: fixed path, or grid-based. Fixed-path TD basically have you spam the path with as many towers as possible, or if you can't make the towers as powerful as possible. Grid-based TD lets you force the enemy into longer runs so they have to go past your towers multiple times and get hit multiple times.
MarZ, by limiting the number of overall towers available (through the use of "hubs"), rather than let resources be the overall limit of towers, basically limited the number of overall solutions available, since there are only 2 upgrade levels (the third one, "specialization", what I call the "plus upgrade", adds very little and is only useful for uber swarms near endgame and only for certain towers, and since tower damage doesn't happen that often, having a repair drone just automates repairs, not enhance combat power). Given that there only are like 5 towers in the game, that's a bit limited.
MG -- your regular gun tower, the upgrade adds more guns for heavier enemies. Plus upgrade turns the triple cannon into a Gatling gun for super zombie gibbing action. No minimum range.
Rocket -- explosive warhead, proximity kills, the upgrade adds additional shots before reload. Plus upgrade adds a mortar for light damage at long range. Has a minimum range.
Laser -- long-range focus on a single enemy, the upgrade adds more power and range. Plus upgrade adds prism, basically cascades onto multiple enemies for slightly less damage, but makes more kills. Has a minimum range.
Tesla -- slows enemies down, and pops tesla bubbles, the upgrade adds range and damage. Plus upgrade adds teleporter: sends one enemy back out of range. No minimum range.
Drone -- sends drones out to engage any enemies, usually one at a time. Upgrade adds drones to the swarm, and each does more damage. Plus upgrade adds minefields. No minimum range.
IMHO, the plus upgrades (or as the game designers call them, "specializations") are not all worth it. The only ones that are truly worth it, IMHO, would be laser, and drone. Tesla's upgrade, teleport, doesn't seem to work on bosses or minibosses, and you'd never want it to be this near anyway. Rocket's upgrade, mortar, doesn't fire often enough to be useful and is often landing in the wrong spot for faster enemies, IMHO. MG's minigun upgrade seems to be a bit too little too late. My credits and tower slot can be better used by one of the other weapons with more range or damage.
When there are "obvious" choices in terms of tactics, tower mix, upgrades, and so on, the design is not as balanced as it could be.
Let us compare it to two pretty classic tower defense games: FieldRunners, and Warstone TD.
Fieldrunners is a grid-based TD with no tower limit. It only has four towers: gun, goo/ice, missile, tesla/laser. But there is no limit on the number of towers you can build, and it's a "build a rat maze" type tower defense game. So you can have a lot of different combinations and path designs. The extended mode adds two more tower types, but still the same game.
Warstone TD has 5 unit types, But you are limited to how many units you can deploy limited to the number of "warstones" you own (and thus can plop down next to enemy's path). You only have 5 units types, but 2 of the units can have different upgrades: peasant > mercenary > knight / witcher > knight templar / inquisitor; and archer > amazon > shieldmaiden / assassin > valkyrie / ninja. So depending on how you upgrade your units, they can do very different things to enemy units. Thus, the tactics will be different depending on your upgrade path, even if you can only place so many units on screen.
MarZ has limited number of towers, AND limited tower choices (with no specialization like Warstone), not path choices like Fieldrunners. So the tactics you can deploy is limited: just build up max towers you can build and max upgrade, and that's about it. No wonder it feels so... limiting.