1) Anything in the castle the players have not taken is fair game for Strahd to use. That includes Sergei's Armor, the Staff, and even the paralysis chest. It's his castle. He knows where his stuff is.
2) Nothing stops Stahd from using the lair action to stay inside the walls or floor and attack through them. Play it fair - Strahd cannot see through stone and needs to peek out / attacks at disadvantage.
Those two combined make Strahd almost unkillable in his castle, especially if the players don't do a good job looting. Just don't have him hang out in the stone too much, things need to stay exciting.
2) Nothing stops Stahd from using the lair action to stay inside the walls or floor and attack through them. Play it fair - Strahd cannot see through stone and needs to peek out / attacks at disadvantage.
Those two combined make Strahd almost unkillable in his castle, especially if the players don't do a good job looting. Just don't have him hang out in the stone too much, things need to stay exciting.