...and that's just mean that Kyros' method to "let just put great magical amplfiers and pretend they don't exist at all, and then nobody will intrested" is, well, flawed. I mean, it don't working. If nobody find that they have some earthbreaking real powers, that's not because cunning Kyros makes everybody thing they're not worthy of exploration.
They are worthy of exploration, granted.
Mages tried to research them, granted.
Wait, what?!
That Kyros doesn't claim and militatise the Spires means ONLY that every mage can come and try to unveal a mystery if he wants. AND THEY DO.
They don't managed to unveil it, but NOT BECAUSE THEY DON'T TRIED. You cited Lantry and Ebb yourself.
If nobody in millenia (hell, what millenia?.. actually Kyros around here 400 years) managed to find real source of Kyros power in the Spires, it means one of two things:
1. It's hidden very well. So well that working of great magic guilds don't unveil anything.
2. There is not real source of Kyros power in the Spires.
Let me highligh it, because I believe it's the point you miss.
If Kyros plan not to forbid Spires is based on idea "If I don't forbid Spires nobody will be intrested in them", said plan is fundamentaly stupid and failed. Nobody reveal mystery not because nobody was intrested. Mages was intrested in Spires, and Kyros didn't forbidding them access actually HELPS research. Said research just don't give any good data, but not because nobody was intrested.
One last time, no-one was looking for the connection between the spires and Kyros unassailble power, the edicts. If you don't know what your looking for, your less likely to find it.
The foundation of Kyros power, is the edicts, that is made clear in game. If indeed Kyros power is derived from the spires themselves or something connected to the spires, it behooves Kyros to make sure that this connection is never made. If the connection is never made, then his enemies can only make assumptions, educated guesses. and can never really strike at the source of his power. Mages can come and go to the spires and the oldwalls, they can study them, ineffectually, as has been shown, the spires have stood for the better part of a millenia atleast and from in game lore, the PC is the only documented case of someone having unlocked the secrets and power of the spires. Except maybe Kyros itself. They have done so likely by accident.
Kyros enacts laws against entering the oldwalls, which in turn limits access to the spires. But doesn't explicitly deny access to spires. Doing so might single them out as important, which is a bad idea. By limiting access to the oldwalls (and by extension the spires) Kyros limits the number of people that might accidently stumble upon its secret source of power. While claiming the spires and and the oldwalls and turning them into military fortresses, all but guarentees a damn sight more foot traffic, more people pouring over the oldwalls and their secrets and the risk of someone else stumbling upon the truth, either by sheer accident or while studying the oldwalls and the spires for their magical amplifiing ability. Fortresses are full of hundreds or thousands of soildiers, mages and more importantly archons, who are more likely to be able to make use of the knowledge they might glean from the ancient structures.
The greatest risk of exposure of Kyros secret source of power, comes from the claiming and militarising of the oldwalls and spires. Tens of thousands of militarily mided people pouring over the arcane secrets of the oldwalls and the spires. Once that knowledge becomes known, Kyros enemies have a greater understanding of his, to that point, unassailable might. Now it's a matter of time before someone finds a means of attacking that source of his power, either to claim for themselves or to destroy the spires outright. This is a vulnerability that did not exist all the while no-one knew about it. The risk that some random mage
might survive a trip into the oldwalls,
might make his or her way to the spire and
might glean enough understanding of the spires to link them to Kyros foundation of power, its edicts, is far smaller and far more prefereable to the first option.
It would be like building the death star with its tiny exhaust pipe thingy, then putting a great big red flag over the port and a ton of guns around it, hmmm the rebel would think...me doth think the innocuous port might be important!....
As I said, Kyros is cunning that way, it relies on people not looking too closely, not asking the right questions and being distracted by the ground that is shaking beneath their feet because Kyros just put an edict on their capital and his armies are swarming over their farm lands, raping and pillaging their people led by all powerful and quite mad archons. It's effective if somewhat risky strategy but far less risky than having the source of his power swarming with the very people she has intimidated into subservience.
Thats my take on it and your welcome to disagree.