After a few moments gathering his thoughts...
Huhuuuurrrm! Well! It seems to me that the spell should be Muto Terram, in order to alter a property of the stone from which our towers are made. A magnitude of 4 should suffice to make the stone slightly un-natural, as it is already stone, if not very hard stone (still hurts if you bang your head on it he mutters to himself).
[This is clearly a topic close to Domazhir's heart, as he is nearly fluent in his speech!]
Aaah! Where was I!? Yes, another 3 magnitudes to allow the magic to affect the entire structure, huurrm! and another for the range of touch, as one is not, after all, a stone tower! (chuckle/giggle/general amusement at his own "joke.")
Hurrrm! Anyway, we have to consider the duration. Aaaah, I favour Moon, as it would give enough time to withstand a concerted siege, without, hurrm! having to run around refreshing the spells every day, while allowing the continued, aaaahm, development? hmmm... of the interiors of the towers as needed by ourselves and the covenfolk. This would require a further 3 magnitudes! Huuurrhuurrmmmm!!
I did mention that we would need 1 magnitude for touch range? didn't I? Ummm! Well? This would lead to aah, 4 magnitudes plus 3 magnitudes plus 1 magnitude (for touch!) plus another 3 magnitudes, making a spell of magnitude 11!!, which is slightly beyond my reach! Hurrm!! But! Together, I'm sure we could achieve it.
We could, perhaps add some magnitudes for complexity to enable changes to the, hurrm! interior of a tower while, ahem, aaah! Yes! the spell is in effect?
[It's actually only Mag 7, but he always confuses things when he's explaining them to others.]