upgrade sketches can be bought in the market.. They allow a player to upgrade high level buildings. for example your resource buildings (farms, lumberyards, and forges) past the level 20. When you reach a building upgrade potential via just resources you will see that more than timber, bronze, and grain is needed to upgrade further. That is where sketches come in.

thanks for your reply OOooBrettOooo, that is a perfect answer :) 

oh yes and the perfect way to get our money............. thank you very much morteeee for you explanation

i knew that u needed the sketches to raise certain buildngs past certain levels but when i finally got to the point where they were needed i was shocked to see i would need THREE HUNDRED of them to raise my agora past twenty. this type of thing is what i really dislike about the game because it gives a distinct advantage to players who have hundreds of dollars to pour into the game. its extremely frustrating when u click on a city and can tell just from looking at it the player must have spent close to a thousand dollars just to make it near impossible to raid their city