(I guess you mean Bc4, jk.) Unfortunately the N on d4 is in the way of Qd5 threatening mate and picking up the black N after Nxe4. Nf6 is a good developing move for black and hitting white's e pawn as Charles comments forces the defence of it. If black doesn't play Nf6 then white also has the choice of playing c5 which grabs more space and control of d5.

I disagree with jkarp, except when he says Black should have played 5...g6. There is nothing wrong in principle with a bishop fianchetto . It exerts control over a long diagonal and helps to clear the back rank. Such a bishop tends to stay put on the kn 2 square for several moves. Its also a key move in some opening systems e.g. Pirc Defence, Catalan and so on. I do this frequently and would not consider that I am a particularly strong player. Have a look at GM games and see how often it is done. Bg7 is a must obviously, when the e7 and d6 squares are blocked. After Bg4 it's too late because of BxN but Black got lucky with a fork on White's Queen and Bishop that allowed him to take the Bishop on g4.

Kurt Paul Otto Joseph Richter (24 November 1900, Berlin – 29 December 1969, Berlin) was a German chess International Master and chess writer. en.wikipedia.org

Leonard William Barden (born 20 August 1929, Croydon, London) is an English chess master, columnist, author, and promoter. The son of a dustman, he was educated at Whitgift School, South Croydon, and Balliol College, Oxford, where he read Modern History. He learned to play chess at age 13 while in a school shelter during a German air raid. Within a few years he became one of the country's leading juniors. en.wikipedia.org

I guess this is one of the points in the game when several choices are possible. I would have normally allowed myself to be bullied by the white B on g5 into playing a3, e6 and Be7. It's an interesting option just to ignore the B and not fear BxN, gxBf6 with the hole on the g file. Would black still castle after BxN or would the K stay in the middle?

Ig g6???? White simply takes the Knight on f6 and black has to recapture with the e pawn, leaving the pawn d6 weak and the square d5 very weak.

CH is right. g6?? would be a bad move. Also, it would be a fianchetto of the DSB on g2. Develop your DSB on e7 or move the D pawn again and place the DSB on b4 with a nice pin. The drawback is having to move the d6 pawn twice in the opening and losing time - to be avoided. Black should challenge White in the centre anyway with e5.

Isn't this walking into trouble, given black's advanced pawn on b5?

You could very well say that. 9. Bxf6 and 9. Nf3 are the usually played moves at this point.

9....b4! is a beautiful move. It advances the Q-side attack at the same time forcing white to retreat his knight. I wonder whether the long castling was advisable in those conditions where black's Q-side pawns were already far advanced?

I think you are right, eddebee makes a similar point.

Yes it was in quick-play. Did you mean 10. Bxf6?

I think this is a hasty mistake (this is a quick-chess match, right?). 10. Nxf6 had been much better in my opinion (10...bxc3 11. Qxc3 gxf6). Yes, Charles, sorry - I meant 10. Bxf6

Indeed!