The key to repairing the castle isn't as hard as it first seems. It's actually all down to simple math, and of course reading the question thoroughly.

The first bit of key information we see is that it costs 74,000 ¢τ a month to hire the architect, and that he would be staying for an approximate three months.

Although this isn't exact it gives us a figure to work with:
74,000 * 3 = 222,000 ¢τ
Plus 20,000 * 2 for the Boat trip both to the castle, and back again gives us a total of 262,000 ¢τ

This is our first step done, now we look at the required materials, and their costs:-

Wood: 70,000 * 23.65 = 1,655,500
Stones: 50,000 * 21.85 = 1,092,500
Marble: 10,000 * 840 = 8,400,000
Tools: 40,000 * 14.85 = 594,000
Total: 11,742,000 ¢τ

Don't forget the discount though!
11,742,000 * 0.95 (95%) = 11,154,900

Add these two major values together and we come to our answer:- 11,416,900 ¢τ
Pretty cheap to repair a castle, Don't you think?

So for the prizes?
Congratulations to...

Tolis
Dogvillie
Gossburg

On getting the answer correct, as well as to everyone else who sent in the correct answer for the last puzzle.
 
These three cities were selected at random to receive the special prize of a Philosopher's Stone.

This item adds +4 to the users reputation points, and can be received without the need for any buildings or statuses.

Out of 31 entries, 27 were correct.