It occurred to me when describing how Citadel Upgrades worked to a friend of mine, that the real, true, problem with most of the upgrades, and why they are only gotten as a money sink, is because for the most part they are, well, backwards.
Consider the one example of upgrades that is NOT backwards: New Creeps. Each upgrade you pay for is (basically) noticeably stronger than the last. Priests greatly increase the power of a wave. Angels a bit more, and Cats and Giants are both huge additions. Shelling out 6k gold for Giants feels about right.
But what about the rest? Giants are the strongest upgrade to creep waves, and are the last to be bought, but for everything else, the FIRST upgrade is almost always the best one!
- Tower Health: Each point in this grants the same Percentage bonus to health and regeneration... but each time you get it, that value is for more money, and is compared against a higher pre-upgrade amount.
- Tower Damage: Like Health, the damage scales by a percent, and is compared to the base damage of the tower, meaning the first point is the most effective.
- Gold: Everyone knows the first is awesome, and the second two are pretty bad.
- Experience: Not only are they small, but getting them later in the game means having less time for them to accrue, and the percentage doesn't increase fast enough.
- Grunt Strength/Defense: These are probably the most commonly bought non-wave upgrades because, of course, they make your giants better. Since they effect the only good upgrade path, it makes sense that they are sometimes bought. Afterall, Giants is 6k, and 10% better giants is only about 1k.
Lastly, buying upgrades from the citadel should have a noticeable map changing effect. It does this for upgrades to the creep line... but not anything else. Towers never get that much stronger, DG's never ressurrect really fast, or gain huge amounts of experience. It isn't like you can ignore the creep wave upgrades, and get tower defense instead, and just hold back the opposing creeps with your new stronger towers.