My intention was that to emphasize the idea that astral events mostly change the severity, or timing of your normal hazards. As I said, my examples are not really very good.
The hypothetical fort would get flooded anyway, the extra water just makes the flooding worse.
The frogmen would raid you every year regardless; the water up-cycle just forces them to invade on a specific day. Only when things have gotten well and truly out of the realm of annual, or even normal events, should really whacked out stuff (like extra elemental creatures) start happening.
I suspect the most useful thing an astrologer will do is pick the most auspicious time of year to forge your admantium weapons. (Whoo! War, metal and craftsmanship are all high right now. It's time to get my weaponsmith on!)
I'd like to see this need to combine with sphere aligned lands, to produce anything spectacular. If you build a fort on a volcano, in a fire aligned desert, on a planet with lot's of firey astral bodies, you deserve exactly what you get, and you deserve some fun fun.
For more sensibly located fortress, I'd like astrology to feel like the man behind the curtain. Are my crops failing because I'm not fertilizing them, or is i the stars? Was my champion's tantrum from poor management, or was it the stars? Did I do something to tick off the humans, or is this siege somehow due to the stars? Higher levels of sparring injury? Maybe it's the stars?
Some stuff should be obvious folklore that any peasant knows. In world with a month owned by death, a farmer should know that plants need extra TLC that month. However, for rarer events, like planetary alignments and comets and supernovas, you should need an astrologer to understand what they mean, let alone predict them.
If a certain event happens every 217 days, once or twice a year, and most of the time it happens independently or in the presence of opposed spheres, the player isn't even going to notice, let alone make a note of how much time passes in between. When that event finally does coincide with aligned events, it's a surprise to anyone who doesn't have an astrologer to keep track of these thing for him.
This was the real point of the earlier example, wherein the player got used to the more predicable elements working in his favor.
"The frogs won't attack until the full moon.
"Oh, Armok! I forgot about the water star!
"Curse you Water Star!"