When They're About to Castle...

When They're About to Castle...

Illingworth
GM Illingworth
Feb 18, 2019, 12:31 AM |
10

...play the best move!

On a more concrete note, here's my puzzle for today! It is White to move in the position below:

I'll post the solution in the comments in about 24 hours, good luck! 
If you're finding this puzzle difficult, follow my guidance from one of my previous posts: https://www.chess.com/blog/Illingworth/diamond-in-the-rough