Consider as a case study three variations on the same type of game:
Each has that familiar gameplay that you know I love: building defenses that blow up armies of mindlessly marching monsters. Each has taken it in a rather different direction. (Each is far longer than the last batch of flash games.)
First, note that each game is “native” to a different site. I have the Kongregate links above, because my life is easier when I stick to a few flash games sites rather than checking all of them, but Desktop TD comes from The Casual Collective, Gemcraft is from Armor Games, and Protector III really is home at Kongregate. The difference is that they have some things disabled off the home site. Desktop TD seems to have the most missing, in the items locked for choose your own TD. Gemcraft locks a few skills. Protector locks some number of maps. So that makes my life harder to the extent that I do not use every flash games site.
These are all sequels, relatively recent ones, the latest editions of some of the more popular flash games around. The gameplay and graphics are largely the same between editions, but things have become more complex over time.
They have different graphic and fiction approaches. Desktop TD is minimalist: circles and squares, getting slightly more colorful and embellished with each edition. You shoot little blobs using towers. Gemcraft keeps the towers but powers them with gems. Its monsters are more varied in appearance, and while the appearance feels less related to the enemies’ abilities, the colors are related to how the receive damage. Protector has fantasy graphics, with wizards and warriors instead of towers.
Desktop TD is the only one that uses mazing. Gemcraft and Protector use static paths for the enemies, where you must put enough damage of the right type along the way. In Desktop TD, changing the enemies’ paths is a major part of doing well. It also creates the option of “juggling,” in which you have multiple exits to your maze of towers with only one open at a time, selling towers and building new ones to keep the enemies walking up and down the paths as they try to get to an exit before you re-seal it.
Protector is the least strategic in terms of placement. The major question is how many enemies a “tower” can reach, with some consideration of leveling up on some maps. You also want to diversify “inflictions” (debuffs), and poison is more useful at the front than the back. Desktop TD has its mazing, and Gemcraft has different abilities on its gems, a stronger version of the infliction/poison considerations in Protector.
All three have progressions through maps. This is something new for Desktop TD. It used to have just the one core mode, with some challenges. Now it has a tutorial built in, with different tower and enemies types being added over time. Gemcraft has standard progression through more difficult maps, with more waves of enemies and more gem types available the deeper you go.
Correction: I should not refer to “progression” for Protector. It is pretty random. The maps have medium and hard labels, which helps guide your progression, but they do not build upon one another. The first map might call for all fire mages, another needs archers, and a third focuses on the melee consecration skill. They are separate puzzles. If they followed the usual progression, you would only have the fire mages for the early fire map, which would unlock your poison mages just in time for a map that needs them, and a little pop up box would remind you about special abilities the first time they are needed. Protector is somewhat less newbie friendly in that way.
Gemcraft adds a new features in this edition: repeating maps under different rule sets. I think this is a great idea, giving each map a variety of challenge modes. One lets you leak no enemies, another makes them tougher, while a third goes for boss fights.
Desktop TD’s version of this is to have multiple modes. You can turn the difficulty up or down. There are a variety of challenges. This all comes together for a build your own challenge mode, where you can add restrictions and challenges, each of which has an associated score multiplier.
“Speed up” mechanics vary. Desktop TD and Gemcraft let you send enemies early, and this gets you more points at the cost of having multiple waves going at once. If you use area effect attacks, this can make it somewhat easier rather than harder, until you leak more enemies than you know how to handle. Gemcraft lets you spend some of your mana to increase how much bonus you get from speeding things up, which also increases how much mana you get in an interesting sort of economy. Protector has a more straightforward button to triple the speed, with no effect on score. Protector also has no option to send multiple waves at once.
Tower upgrades also vary. For Desktop TD, it is simply a matter of money, plus a few moments in which the tower closes for construction. Gemcraft has both of those, plus you merge gems together and can decide to upgrade one purely or in combination with another color. Protector requires gold but no time to upgrade, but it also requires experience points. Each wizard/warrior gets experience for damaging enemies, and it must get a certain amount before you can upgrade. This creates a need for strategic upgrades on some maps, because you may want many level 3s rather than one level 6.
That is another difference: Protector often favors many low-level towers, while the other two favor fewer high-level towers. The need for experience and a relatively small gain for leveling combine to encourage many attacks rather than a big boom. Desktop TD encourages many towers mostly to the extent that you are creating a maze, usually with many cheap towers and a small number of primary killers. It is easier to cap out your tower level on Desktop TD than Gemcraft, in most maps, so Desktop TD tends to see more towers at its lower cap, while Gemcraft does not see masses of maxed towers until very late.
Protector’s leveling up includes the unique mechanic of heroes. In all three, everything resets between maps, but Protector now lets you buy a few wizards or warriors that keep their experience between maps, and you can level them with leftover gold. They are more expensive to start, but they bring larger destruction and special abilities.
Protector and Gemcraft share another leveling mechanic that Desktop TD lacks. Your score on each map adds to your experience in CRPG fashion, letting you unlock skills for future use. This is a feature that returns from the previous Gemcraft, but it is new for Protector. They also mesh with a sort of achievements system, providing more experience or other bonuses.
They implement this differently. In Gemcraft, what counts is your highest score on a map. If you repeat a map and get a newer high score, it subtracts the old high score and adds the new one. Accomplishments are on a per-map basis, with a few global, adding to your score on that map. In Protector, you can repeat a map for more experience. If you want to grind, go ahead and grind.
Personally, I do not favor this mechanic, although I think it works better in Gemcraft. Because the first Gemcraft uses this mechanic, as with other games from this creator, it is built into the structure. You advance through longer and more complex maps, and you have more toys by the time you get there; if you want even more toys, repeat the older maps for higher high scores. Protector, contrarily, does not build on itself, nor does it expand your toolbox as you approach each separate puzzle. Leveling up just makes your tools stronger, and its achievements reinforce what you have already done over encouraging new things. If you are having trouble with a map, have you not figured out its trick, or do you just need to get more skill points/achievements/hero levels to beat it? If the maps are balanced with the assumption that you will have leveled, they become impossible without some grinding; if they assume just the base level, maps become trivial after grinding. Neither is a good option. It is only made worse by making it uncertain whether the content is gated.
Gemcraft adds bombs, giving you something to do other than build towers and traps. At any time, you can detonate one of your tower gems on the enemy. This helps you stop monsters at the last moment or deal with multiple waves at once. Gemcraft has a few other wrinkles, like towers versus traps and new structures that keep you from building in some areas or give you bonuses.
Desktop TD and Protector give you limited numbers of lives. If that many enemies make it through, you lose. Some bosses in Protector have a special ability that sends them back to the entrance for another trip through, which gives you another chance to kill them for the xp/gold at the risk of losing multiple lives. Gemcraft has no lives, just the mana pool that you use to summon towers and gems. If an enemy makes it to the end, you use some mana to banish it back to the beginning. Dead enemies give you mana, and mana increases over time. Your gold supply is your hit point supply.
Desktop TD and Protector share the usual mechanic of giving you money for each enemy killed. You can also get an upgrade in Protector that rewards you for money unspent between levels, as is common in tower defense games. Desktop TD gives no reward for using the minimum force: spend all your points, now. Protector lets you carry some gold between levels, in addition to the interest upgrade. Gemcraft gives no reward at the end for remaining mana, but you get bonuses for sending enemies earlier and you can spend your mana to increase that bonus, so you pay up front instead of trying to accumulate an unused pool of liquid resources.
A bonus fourth game, Bloons Tower Defense 3, is a less recent but also popular tower defense game sequel. Bloons uses pathing rather than mazing. It rewards for each kill, plus over time, plus interest between waves. It has relatively few maps, but you can set the difficulty on each. Like Protector, it has no progression in that you have all your towers available on every map. There are no levels, and nothing carries over between maps. It has a surprising variety of enemies given that they are a bunch of balloons, and gameplay puts a heavy emphasis on the effective use of area effect attacks. It has an unusual take on towers and monsters: monkeys throwing darts at balloons, although that can upgrade to catapults throwing big spiked balls at lead- and ceramic-coated balloons. Bloons TD is native to Ninja Kiwi.
Full disclosure: if you use one of those links and sign up at Kongregate, I get some kind of points that do nothing but put a number by my name. But, you know.