Continuing further in Prime World: Defenders has answered some questions.
Yes, the rarer towers are unambiguously better than the basic ones. Early on, my mostly upgraded wooden tower was boring but practical. As a great lover of DoTs, my fully upgraded poison tower is awesome yet practical, but the better towers are generally of the rarer rarities, and they can be upgraded further from there. I am told that the dragon tower is especially excellent for the mid-game, but I have yet to find that card, instead finding multiple sun and lightning towers (crushing individuals and groups, respectively).
Yes, the later levels are balanced around a resource grind. You must complete side missions to collect and upgrade cards, or else your numbers are not big enough to complete the main maps. There is a daily login prize for some reason (currently disabled by bug), so you could theoretically just wait instead of grinding, and you will be ready for those later maps sometime in 2015.
This game reinforces the critical importance of threshold values. An attack that does 50% of an enemy’s hit points is worth about as much as an attack that does 90%. Not exactly, because you can mix heavy hitters with fast or AE attacks, but there is a huge difference between 99% and 100%. This is where the better towers and upgrades become critical.
Oddly, the game has some anti-farming code. I found the 4th boss trivially easy and repeated it to see how quick I could complete that map. The 4th boss is now worth 0 silver and 0 xp. You are supposed to farm, but you are supposed to farm the random missions. I have reached the point where I need to farm the easier random missions to beat the harder random missions to farm rarer cards to beat the main missions.
Hmm, maybe I just need that much farm for beating it cleanly. I am caught in the circle of farming. Back to the regular mission chain! [Update: nope, needed a little more farming. On tower defense, the margin between “can’t beat it at all” and “can beat it 100% perfectly” is small.]