Complex sentence: The existing fortresses had fallen in to a state of decay and it was decided the Prince of Wales and Prince Albert Batteries were inadequate for the defence of the town and in 1871 work was begun on another battery but it was stopped when funding ran out.
Split into simpler sentences: The existing fortresses had fallen in to a state of decay and it was decided the Prince of Wales and Prince Albert Batteries were inadequate for the defence of the town. As a result, in 1871 work was begun on another battery but it was stopped when funding ran out.