Fortress Sudoku (Daily Sudoku League #67)

Sometime back I posted Poor Neighbours Sudoku as New Sudoku Variation. That time I also created a reverse version of this variation and named it as Rich Neighbours Sudoku in which the number in the grayed cell is greater (Richer) than its orthogonal Neighbours. During the testing of these puzzles, Prasanna pointed out that Rich Neighbours Sudoku is nothing but Fortress Sudoku with the only difference being that no two greyed cells here do not touch each other. I completely agreed with him and don't want to create a new name for an almost similar Sudoku variation. So I am posting this Sudoku as Fortress Sudoku.

I am posting on Thursday for The League of Extraordinary Ladies and Gentlemen, however, today Prasanna is not able to post. Maybe he is busy with relocation. I am posting on his behalf. This Fortress Sudoku is the only puzzle among my set of puzzles that I have kept in reserve for the Daily Sudoku League which is tested. All other Sudoku puzzles are still under testing. Hopefully, at least one more of my Sudoku will get tested when my turn comes coming Thursday.

This Fortress Sudoku is my 67th contribution to The League of Extraordinary Ladies and Gentlemen. Can you solve this Fortress Sudoku puzzle?