The higher difficulties aren't really strategy, just luck. If you hit any sort of impediment, like a bunch of barbarian raids, or iceages that slow down population growth, it becomes mathematically impossible to win the game.
There seriously needs to be a set of forced action commands.
It's extremely aggravating when the workers just dither and ignore much needed tasks, for instance I'll be constructing sniper towers to head off a wave of enemy. I'll have plenty of free people to man the towers, plenty of free workers to build the towers and man the towers, plenty of assigned workers to the nearest supply depot with a ready amount of supplies at hand and more than enough time to get the job done. Yet the workers just ignore the build order and the area gets over-run. Going by the in-game wave clock I've had build orders take a full two minutes before any construction finally happens.
Another necessary forced action is bloody retreat command. When I know a branch of my base wont survive a zombie attack I raze the buildings in order to pull the people back and keep them from being killed. Instead they just stand there doing nothing until the zeds over-run and kill them.
@silverfalcon07: Same, massed greenjackets just mow down anything that come within their considerable range. After getting them stacked three to four deep the engineers can't even touch the walls. Something that might add some challenge is if the opposing force could roll in some field artillery or mortars in the later stages to keep you on your toes, it would also force some development of your own fort artillery as a counter.