New York City's autocratic numero uno Rudy Giuliani is pressing ahead with plans to build a mayoral fortress in the World Trade Center, a bulletproof bunker with a hotline to the White House. But the project isn't getting the respect a real grown-up $15 million command-and-control center deserves. Lefty lawyer and radio commentator Ron Kuby has dubbed the facility the "Nut Shell." The New York Daily News is comparing it to Saddam Hussein's bunker, and other detractors say it will cost tens of millions of dollars more than Giuliani projects. Yet the mayor who squeezed the city's squeegee men is unlikely to give up his castle in the sky over a little ridicule. Perhaps he's just spooked by all the sci-fi movies depicting his glorious city being swamped by tidal waves, squashed by meteors and incinerated by alien space ships.