Blessed be YHWH,
For he has shown me his marvellous covenant love in a strong city.
As for me, I said in my haste,
I am cut off from before your eyes,
Nevertheless you heard the voice of my supplications,
When I cried to you.'
So he blesses YHWH who has revealed His marvellous covenant love towards him ‘in a strong city' where he can be safe, and that in spite of the fact that in his desperate haste he had said, ‘I am cut off from before your eyes'. For YHWH had graciously ignored his despair, and had heard the voice of his supplication.
The strong city may be Jerusalem, and this may therefore refer to David having come through all his trouble to find himself now established in the stronghold of Jerusalem, or alternatively it may be indicating that God Himself is a strong city for those who trust in Him.