IMHO the best way is join a brotherhood, they help in so many ways including sketches or you can buy them in the smugglers den.

As for fortifications, you can go 0 to 5 or 1,2,3,4,5 in the end it will cost the same

All the best

Andy 

You can get sketches from other players, they can send you them or you can trade with them.

About fortification, you can wait and build highest level possible, you can build different levels at the same time.

The important thing is that you can upgade your current fortification as soon as you finish reasearch of another level.

The price will be corresponding, for example, level 1 wall is 50 rubies, level 2 wall is 130, thus upgrade from lvl 1 to lvl 2 costs 80 rubies.