Complex sentence: Towards the end of the seventies, this concept wasn't successful anymore, so the buildings of the castle (especially the gatehouse of the upper castle and the entire lower citadel) were neglected more and more and fell into disrepair.
Split into simpler sentences: Towards the end of the seventies, this concept wasn't lucrative anymore. Therefore, the buildings of the castle (especially the gatehouse of the upper castle and the entire lower citadel) were pretty much unatended and fell into disrepair.