This is remarkably similar to the game of go. The thing is that the devil can place pieces anywhere. They don't have to be close to the angel. So he really o

This is remarkably similar to the game of go. The thing is that the devil can place pieces anywhere. They don't have to be close to the angel. So he really only has to calculate the arc that is possible for the angel to travel in and play pieces somewhere around there. As the angel gets closer, the devil needs to fill in the spaces between his pieces. But because he doesn't have to construct a complete wall, his initial moves are very inexpensive. A single piece can be used to potentially block off a fairly wide arc. The trick is to progressively restrict the options of the angel until there are none. If the Angel backtracks, this is good for the devil because it's amost like getting a free move. You can imagine that if the angel moves right and then moves left, he's back in the same place but the devil has place 2 pieces. So the Angel is a lot more restricted than it first appears.