On my 100th year of my dynasty, I was able to build a Magnificent Palace.
It's rubbish. The most fun you can have is with the cannon tower (and I was in a position within the council that I could make the cannon tower legal). The Alchemist will not work, and when he does the stuff he makes is not worth selling.
So remember my Guild-loving children, The Mag Pal is not a good investment. Unless you like to blow shit up (which is in itself quite costly).

Who doesn't get a kick out of massive explosions?

The explosions aren't that impressive

.. What if you want to just be able to say..
"I own a frelling castle, biatch!"? :)

I actually found the cannon tower to be VERY useful.
If a property that I want to buy is obscenely expensive (generally those owned by a rival dynasty, they will refuse to sell to you unless you give them at least 4+ times the shop's actual worth), I unleash my cannon on that shop.
Muahahaha, when the shop is damaged until the worth of it is just a fraction of what it was originally, when I make an offer for it, the owner will sell it to me rather cheaply. [evil grin]
Of course you have to renovate it after wards, but considering how much the owner was asking for it, the effort is all completely worth it in my opinion.
Also, since I am holding the position of sovereign on the council, I get absolute authority of when using cannon towers are legal. Therefore, I make it legal when I want to use it... and make it illegal when I don't.. [evil grin x2222....]