The entrance to Portsmouth Harbour, showing the The Round Tower and the Square Tower - with the "twin" Round Tower (long since consigned to history) on the Gosport side of the narrow strait, any unwelcome ship trying to sail into harbour would have been blown out of the water in minutes.
The addition of the huge chain, which could be raised across the narrowest part of the channel, also meant that any unwelcome ships would literally grind to a halt, right in front of the shore gun batteries - both the French and the Spanish would have been well aware of these deterrents and, needless to say, no belligerent ship's captain was ever foolhardy enough to risk trying to enter the harbour.