if you cant castle after nb4, kf1 works quite well. Just remember to steal a tempo by attacking their queen with your dark bishop if its possible to avoid nxa2 and nxc1 if there is no light bishop defending the a pawn
Nb4 sacrifice explanation
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Maybe you have already seen a knight getting sacrfificed on the b4 square. Today I"m gonna try to explain how this attack exactly works and how it is stoppable.
Imagine this position :
1. e4 c6
2. Bc4 e
3. c3 Qc7
4. Qe2 b5
5. Bb3 Na6
6. d4
Here black is able to play the move : Nb4 !
The idea is the following : once white takes your knight, you're gonna play :
7. cxb4 freeze@d2 Bxb4
Then if white doesn't block the check and do something like :
8. freeze@c5 a3 jump@c6 Qxc1# And you've won.
If white blocks with something like :
8. freeze@c5 Nc3 Qxh2 And you'll be up an exhange.
If white doesn't take your knight, you will be able to get a freeze tactic on c2 or d3.
To get out of those Nb4 sacrifices, you need to be able to castle out of the way when the knight arrives on b4. It's why you need to quickly develop your king side's knight and bishop for white in those positions.
Summary : to get the Nb4 sacrifice for black, you need white to have moved his d pawn (here d4), get your queen on c7 and white not being able to castle 'cause your sacrifice would be unuseful and white would just O-O.
@Delhy