Storming Dooms castle? Then beat Doom? errr idk not many can beat doom in his own castle. Magneto tried and He himself is a genius and a great straegist (prolly not as clever as lex or as coniving) and well above the power of Lex and whatever suit he has; and he failed. Lex would have to use every trick in the book from orbital satellite lasers (think Lex Corp has that) and a army of robots or some jazz like that. So doom and his castle ftw.
Just look at it this way: If Doom were being attacked in his castle by the Avengers (any decent roster), would he be worried or laugh menacingly and prepare his defenses. Clearly the latter, and he wouldn't plan on losing (for good reason).
Now, Lex is smart, but would he be > Avengers? Doubtful.
Doom doesn't need to know he's coming...his castle defenses are years of prep pre-built.