atulab: note that at leaves a ko and that omitting prevents black from protecting the squared stone
xela: This is not the solution. Black plays a and white is totally dead.
A KO fight seems to be the only way.
(White will need to find the first KO threat) If White will loses the KO, Black will get the marked White stone and get two eyes in the corner -- forcing White to escape.
The circle point is a threatening move for White to escape out through the Black stones.
Black played at 2 to prevent a White play at Square having a follow-up move for White at 2 (and squeeze Black's liberties).
This White 1 could get more liberties on the outside then Black can get on the inside, so Black needs to prevent this.
This assumes that Black desires the KO fight rather than claiming the corner and letting White escape so Black can have sente.