You have to destroy the ward to be able to build or repair anything in the tower, but you should definitely replace it with your own ward to improve the buildings' resistance to attack.

You can build your black forge (and other infrastructure) anywhere you like, as long as you plan ahead enough to have the materials on hand in your chosen location. You'll need to import bronze, black metal, and possibly copper to properly make use of the forge though, and you'll need to bring in 10 tin for a cauldron if you want to do a full base in Mistlands.