In Old English the word beorg/beorh meant "hill" or "mountain", while the word burg/burh/buruh or byrig (dative case) meant "fortress" or "fortified town/city". I'm not aware that the former word survived into Modern English in any form ("iceberg" is believed to have been borrowed from Dutch). The common Old English word for hill or mountain was dūn, which survives in place names containing "Down" or ending in "-don", and also in the common adverb "down". ("Dune" was borrowed from Dutch.) The various forms of burg survive in numerous British place names ending in "-burgh", "-borough", and (from the dative case) "-bury".
I don't know what "Heisen" means. If it were "Eisenberg" it would mean "iron mountain", but I don't know that the Germans are as cavalier about their initial aitches as are some other languages.
I dont quite know what Heisen means either and I am German. It might be some old long lost word.
It could be an old form of Eisen, but I doubt it.
Eisberg is the german word for iceberg, btw.
It is pronaunced the same way iceberg is in English.
You know, I don't think I ever learned what the German word for ice is. Is it Eis? That would make sense, it would be the same root, just spelled differently.
Of course I could look in an English-German dictionary, but that would require not being lazy.