Well..... since no one else has answered you I'll give you MY opinion. Its only that too, an opinion. I don't think there are any hard and fast rules about secret knowledge. Its extremely important for you to think of your .... future..... in the game. Do you think, really think you'll be here for the long haul? If so, DON'T leave the level 8 troops catagory until last. It can get stupidly expensive if you do. Yes, I know THAT particular pain lol. I went with the "big 3" first. Offense, Defense and Health. Then after that I think its more of a players ..... style as to what to finish up with. Will you be a hyper aggressive player? Then the Towns section should be high on your list. The Fortress II will help with Sieges you defend as well as Asgard etc.  I've already mentioned the level 8 Troops.

I have 2 accounts, both level 40 Palaces. Am almost completely finished with Knowledge on both accounts. I seem to have had an easier time with my second account as far as knowledge is concerned but thats just because I learned from my mistakes with the first one lol. (see my comment about the lvl 8 troops earlier lol) 

Take a minute and really reflect on your playing style, your goals in the game and how much time , money or both you intend to spend here. 

Good luck in the game and I hope you have had and get as much enjoyment from it as I have.