How did Japanese Occupation influenced the view 'Singapore as an impregnable fortress'

War came swiftly to Singapore, 8 days, despite of the big guns that protected Singapore's coast. The Japanese knew that the big guns protecting Singapore's coast made the invasion by sea difficult and dangerous, they came by land from Johor. They had a very good strategies and came prepared. The Japanese also sent spies to Singapore. Britain did not send adequate reinforcement to defend Singapore as they had to concentrate most of its military forces in Europe and North Africa, Singapore was not their main focus. The British had a lack of weapons, and they were not prepared.The swift loss displayed immediately that Singapore was not an impregnable fortress any more as they could not protect Singapore from the Japanese.