Some Pit Fiend owns the tower. While the Infernal Armies were intent on building the tower, some Pit Fiend functionary signed his name on the deed and filed it somewhere in one of Hell's many Deed Registration Offices. So until Lord Hex (1) finds out where the deed is, (2) looks at the deed, (3) finds the Pit Fiend, and (4) convinces the Pit Fiend to let him in, Lord Hex cannot enter the tower! Sometimes being a vampire is more trouble than it's worth...
If you want a simpler, owner-less solution... the devils once had a problem with undead entering their tower and invading THEM, so they cast a powerful spell that keeps undead out. Lord Hex will either have to (1) become alive again somehow, or (2) use some non-undead (ie: the PCs?) as catspaws to remove the item that powers the anti-undead shield.