I am glad, ydejin, that you started a thread on the problematic subject of casting strategic spells on a city.

I reinstalled the game, yesterday, and started a new game. My capital had grown to many tiles ("squares"), on the map. My Sovereign, an enchanter who had 15 mana and who had learned 2 strategic city-enchantments, was located within the city. I opened the spell book, like the previous poster wrote, but the cast-spell button was not there!? I moved my Sovereign outside of the city, next to it. The button somehow became available.

But after you press the button, there is not enough visual feedback (such as a special animation). After I pressed it, there only was a light visual cue : when I passed the cursor over the city tiles, a faint, red glow was visible. If I then clicked, nothing happened : the red color meant I was not allowed to do it, even though I was in range. I finally found ONE city tile which became covered by a light, green glow : I clicked on it and the spell was cast. 

Perhaps there was an easier way to do it, but I had to struggle for 10 whole minutes -- by trial and error -- to get it right, not really understanding what was the normal, correct procedure. Why did the casting button originally not appear on the spell, suddenly appearing a few minutes later?

It is not a simple, obvious, "intuitive" procedure -- and the manual does not show the procedure. Many new players, who have not participated in the beta, will probably experience the same difficulties you and I have had.