- The London as well as the Stonewall to a great degree are based on ideas rather than exact variations and involve a lot of possibilities for transpositions and move-order tricks.
- In both openings you normally attempt to fortify a central bastion rather than create immediate central activity.
- Both openings frequently allow you to switch to a kingside attack shouldn't your opponent play actively enough.
- Both openings lead to characteristic pawn structures that can frequently be recognized even in the endgame.
- There are certain London lines where White sets up a Stonewall formation (normally after the exchange of the light-squared bishops but not exclusively).
- Both openings are in my opinion a bit underestimated.
A typical Stonewall position:
See the similarity?