Open these secret doors and find plasma gun ammo and a 1-up (you may want to save the 1-up). Find a health chalice behind the wooden barrels to the west.

Find the silver key here. Beware the guards.

With the two remaining crosses you may take any two...

...of these...

...or these death chalices. If you take the two latter you may now continue east and clear the way through the vine cellar to the dungeons. Don't get surprised by the enemies lurking among the vine shelves.

To clear the way to the exit up north fight your way through either the green or the purple adjoining room. Shoot any of the barrels. Open the secret door. Beware the bomb, and the armour if you have not already finished with it.

Continue.