You cant open the tomb in current state of the game, its impossible. But if you go outside the tower (to the maze) then pan your view down to the NE and you will see a skeleton cave on the beach where the dragon is (if you explored). Click the skeleton cave and boom, your characters will automatically run inside the tower, down to the tomb and appear outside the skeleton cave (just sit back and watch it happen). So i assume its just a broken tomb for now but the intended effect can be reproduced by doing what i mentioned to get forward. Unless you have enough teleportation gloves/spell/scrolls.