Return to Dark Castle is COMPLETED, according to Z Sculpt Entertainment. In order to release the game, however, Delta Tao Software needs permission from Mark Stephen Pierce, the original creator of Dark Castle and the current owner of the rights. Contact them if you want to see this game released, the more interest they see the more likely they'll get their act together:
Joe Williams: http://www.deltatao.com/contact.html
Mark Stephen Pierce: http://www.superhappyfunfun.com/contact.html
Thanks!