Oh yeah, for the record, this is a hard game even on 'normal' settings.
It's not overly complex, but one small break or weakness in your defense will quickly kill/infect large portions of most city setups. Your civilians are stupid as can be and do NOT run fast enough to not get immediately infected if you don't wall them in and defend them appropriately.
There's definitely an optimal build path and defensive setups; part of the fun of the game for me though is finding those out, albeit the hard way usually. Also some aspects definitely seem to be a little different from the last time I played, AND in the campaign so far I'm missing units and tech(assuming they get unlocked later).
No D towers early on = much harder in the campaign early game than even the sandbox mode, honestly. No option but to spam rangers and walls.
Micro-ing groups of ranger to clear random standing/walking zombies before the main rushes comes is required in any game mode also. Otherwise they ALL come at you later.