There are many challenge sites on the net, and one to unify them all.
I decided to focus on hacker.org as there are many high quality challenges involving programming, cryptography and maths, and a bit less of ugly exploits and similar. The site organizes challenges on a map. Harder challenges are on mountains, easier ones on grass. Each time a challenge is solved, it opens the road to new challenges. Castles protect challenges that are on crossroads. Winning against a fortified castle opens new possibilities and new areas to be discovered.
From easy to hard or the other way round
Sometimes i get stuck solving a harder challenge. At first time, it looks like i am wasting my time, but the point is that if by chance i manage to solve it, several other challenges suddenly get much easier.
This was the case with the Labyrinth challenge, which forced me to develop new techniques for the HVM. In the end, I invested more than one month of time on it. Once Labyrinth was solved, I could solve the similar Deluge and the sequel "Flash Flood warmup" in one day. Other challenges like "King Mouse", "Brokenest Keys" and "Mus Minutoides" finally found a solution through new ugly techniques (mainly calls with no returns and what i call the statistical approach, to spoil them a bit).
To sharpen my weapons further, I put Caesum's Challengers handbook in my journeyer's backpack. And here I found a good explanation of the chi and kappa test, so that I added the index of coincidence to the magic spells I am aware of.
It was a great satisfaction to besiege the castle protected by 'Spiral bits'! The path following algorithm did his job well, but the decoding part was a complete nightmare, so that i turned the Lazarus GUI into a complete editor for inserting, deleting and flipping bits. I also had an integrated lens on board and a tool to compare the decoding on slightly different paths (which did not work very well though). And a good bit of luck was also part of solving it!
I then succesfully attacked 'No full ACK in HEX' and 'No full ACK in DEC', learning Knuth arrow notation, the tower of powers, and in particular learning Python's ability of computing with large numbers, which is unrivalled. Maple couldn't do what Python did. Now I know why some numeric people I worked with were so happy with Python :-)
And still, there is work to do. My armies failed several times against 'Blizzard' and 'Descartes'. The 'Captcha' prototype needs further refinement as well...
(current ranking on hacker.org is 188 with 180 challenges solved/on Khan 200 challenges solved and Copernicus badge reached, next goal is one million points)