Your task is to make your way from the Cripplegate to the Shard. So ride the Fortifications Line until you get to Bishopsgate, then get out and walk south, turning at that weird blue box. Clear?
If the ancient Romans who built London (then Londinium) had also built a subway line to match their roads, then a map of the network might look like this. You can find a larger version at The Londonist.
-via Nag on the Lake