At last I am free to speak of some things behind the curtain; I can say without equivocation that the crossdimensional door is gone, and the players will not be able to change any of the things I'm going to say now.
I hope you will forgive me for any railroading with the door; I'd like to assure the party that there was a non-door option, but it almost certainly would not have been seen as the desireable one.
My plan, if the party did not take the door, but were instead ready to fight the six marines assaulting the tower, was to have the following occur:
1) To have Nachnamen, the officer left outside the caverns, to make his first appearance on the scene - he would have started, with his men, upon the Worgs below, making it possible for the party to return to the interior of the castle.
2) Once the party had been fighting the men on the tower's crown for six full rounds, on the seventh round I intended to introduce a 7th level fighter, teleported into the tower by one of the mages below on the party's side, who had ALSO realized the Stinger was not functioning. Determining that a fight was going on there (augury), the fighter Wainscomb was going to be there to back off the men. All things going well, the party would have lived and Nachnamen's party would have helped heal them, to keep the battle going.
This was, of course, not revealed to the party in the slightest. It was my intention that the party would need to commit to a moment of possible death - such things are dramatic, after all.
As surmised, yes, the location of the room was entirely random. It was jumping to those areas where I had mapped, and for which I had area data (updated from the wiki posting), a possible 13,500 hexes (661 of which are in Germany). For interest's sake, the locations the party appeared (barring this last one) were the Muyun Kum desert, 100 miles north by northwest of Taraz in eastern Kazakhstan; central Moscovy, 40 miles West by southwest of Peroslavl-Ryazan (about a hundred miles from Moskva; the coast of Sivash, 15 miles due east from where the Crimean Peninsula meets the mainland; and finally the Sam-al-Katub desert, about 30 miles from the western shore of the Aral Sea.
If the party feels that they've been treated unfairly, please tell me. As it happens, its quite a comfortable way for a DM to wrap up a rather difficult battle endgame ... and after all, you did live through it.