I haven't played that many Tower Defence games, but in terms of gameplay, this has the usual set up: a variety of towers you can place down in designated squares, waves of enemies spawn in, and follow a set path, taking a barrage of your bullets/lasers/missiles along the way. If they reach the end of the path, you lose a life. Lose all 10 and you have lost. You gain credits per enemy killed and can buy more towers or upgrade them.
There are 3 special powers you can use multiple times in the level, but they have a cooldown period so need to use them wisely. There's "Slow down", an area of attack "Super nova", and "TB2 Aircraft" which seems to specifically target the exit.
I was having fun until I got to level 4 then seemed impossible. I saw a review that mentioned you could go back to improve your scores to get more upgrade points, but when I was awarded 4 points and I was sure I got 3 the first time; it made me realise you can actually just replay levels to grind upgrade points. This means the system is essentially pointless. At the time of playing level 4 I think I had 8 points, but with another 4, it made a big difference.
The upgrade system is a bit weird anyway. For each tower, you have the predictable ones that boost attack, fire rate, range, and then there is buy price and sell price too. Once you have upgraded the buy and sell price, you can actually be in a situation where you can just place a tower down and instantly sell it for profit which is incredibly stupid. The other stupid thing is that the other types of upgrades are a percentage chance to place an upgraded colour tower.
The basic towers are blue, then green, red, then gold. In the game, once you place a tower, you can pay credits to upgrade them, where every 3 upgrade levels upgrades the colour. Each of these upgrades is extremely pricey, and progressively and exorbitantly expensive. They are basically unobtainable during the earlier levels. The price doesn't change for the colour, but changes by how many times you upgraded it. So it seems upgrading a new blue tower by 1 level is the same cost as upgrading a new gold tower by 1 level. Since the colour of new towers is random, it means your performance is at least slightly weighted by sheer luck. When you have limited upgrade points to assign, the system encourages you to specialise in certain towers since guaranteeing upgraded towers makes a large difference.
The thing is, since you can put upgrade points for a chance to place down an upgraded tower, the cost seems stupid in comparison. When you can get the upgraded colours for free why would you want to spend 10,000+ credits? If you only have a few upgrade points placed in the percentage chance - you could even place down a tower, then sell it, then place it down again and hope you get the upgraded tower (and repeat). Even though you might be losing money, it is still cheaper when you might just lose 200 per time until you get what you want, compared to spending thousands to upgrade from colour to colour.
The towers themselves are quite imbalanced. The Rapid tower seems fairly effective with its speed, and is devastating once you upgrade the damage. The Flamegun seems weak and a bit slow, and even when you upgrade it - it pales in comparison to the other options. If you test out a group of Flameguns next to a group of Rapid guns you see a huge difference. The Source towers cost 1000 and then give you a meagre 11 credits per 4 seconds which means it takes ages to get a return. I’m not sure it was a bug, but they don't seem to generate anything until you place down 3 towers, and buying one of these early puts you at a disadvantage because you lack the firepower to deal with early threats. Yet, buying them later makes them more pointless. Upgrading them makes them more beneficial but still takes time to recoup the initial outlay as the cheapest you can get them for is 800. When you have enough upgrade points to make them worthwhile, they aren't worthwhile because you can just have extra firepower and cheaper towers instead. Plus you can exploit the buy/sell mechanic for much greater gains.
Rocketguns seem way weaker than the game claims, but I found them useful due to their range. With your strong Rapid guns doing damage as the enemies come in, many enemies are wiped out early which means towers behind don't get much action. But if those towers are Rocketguns, then they can launch to where there is action.
The waves of enemies seemed to have a strange difficulty spike. I often found I had a good strategy, evidenced by the fact that no enemies made it past the halfway point. Even as the frequency of enemies ramped up, they still couldn't make it through. Then, out of nowhere, usually with about 4 waves to go, a group of strong enemies would get through and make it all the way to the end. So my defence could fend of hundreds of enemies before the halfway point, then all of a sudden a group gets through those… and all the other towers. It's weird it seemed a consistent occurrence between levels, because I noticed when I restarted the same level, I was seeing different enemy patterns in the first wave. So there is obviously some randomness to the waves.
Entrance and exits aren't marked which is confusing on a lot of levels. Sometimes I clicked play and waited for the enemies to come in before placing towers. I'm not sure on the exact reason, but I'd often lose sight of the cursor, and even wiggling the mouse sometimes wasn't obvious where it was. The small buttons that appear when clicking on your tower to upgrade or sell were too easy to miss when clicking - it's almost like the hit box isn't big enough.
A recent patch put out by the developers' was to address "balancing issues" and fix a bug where the game fails to trigger the end state. I ran into that end-state bug, and thought the game was ridiculously unbalanced. I think one change they made was that the initial 3 towers you place are always Blue, regardless of your upgrades. I think this was to stop people placing towers and selling them for profit before you have even clicked the button to start the level. I quite liked that you have to place 3 basic towers down first, but maybe the random chance idea shouldn’t even be a thing.
I think there is something inherently fun with Tower Defence games. Just watching all the projectiles and explosions is satisfying and it did keep me entertained - I mean, I did play it for 11 hours. However, it is clearly made by developers that know nothing about game design and therefore it is hard to recommend.