You are correct, there are Spires all across Terratus. The exact distribution of these monuments fuels all sorts of debate and speculation, but two facts seem commonplace: most Spires are built near Oldwalls (your Mountain Spire is a notable exception) and nearly every culture that predated
Kyros has some local myth to explain the builders of these Spires - I believe the Tiersmen cite the "Older Realms" as their local name for the Spire/Oldwalls architects... but nobody knows for sure.
And of course, where
Kyros has brought proper civilization, the origin of the Spires simply isn't discussed in good company...
Here's what I think you need to know...
First, the Spires behave as arcane lodestone - there is a constant field of magical strength around a Spire. If you imagine a mystic sea resting atop our world, the Spires seem to spin and churn this magic - creating a spell cast with even modest effort will materialize with powerful results.
Second, and I'd say most tellingly,
Kyros has laws against entering the Oldwalls, but there are no laws against the Spires. You could infer from this that means
Kyros has no issue with the lay folks messing with the Spires but wishes to keep us safe from the Oldwalls. Or you could think like an Overlord and know that if you place a rule against something, you make it forbidden and desirable... now I find it telling that
Kyros forbids the hallways that leads to the Spires but does not paint a target on the Spires themselves.
Last but not least... as a young Fatebinder, I once had to oversee the case of a man who had built a shrine to
Kyros around the base of a Spire in the far south east. I could find no evidence of falsity or slander - his shrine was a most reverent and loving tribute as I've ever seen. At one point though, this man claimed he had evidence that '
Kyros was here back in negative fifty.' He had merely uttered those words and my senior Binder swung the axe then and there. Being the new girl at the Court, I said nothing, but I have long wondered why such a comment should require death instead of the traditional removal of the tongue.
-Fatebinder Myothis