In the early 1800's merchants and other influential people of the area thought the Port of Liverpool should be guarded, so when the old perch Rock Light was once again washed away the authorities considered having a fortified lighthouse, or a fort which would contain a lighthouse. At a meeting held on the 25th of March 1825 they decided on two separate constructions, a Lighthouse and a Fort. 

The Lighthouse was originally called the Rock Light, but it has had other names ranging from Black Rock Light to Rock Perch Light, it was in the 1870's that the name Perch Rock Light became its commonly used one. The name Perch Rock dates from before the "Lighthouse" was built as the outcrop of rock, known as Black Rock, was a hazard to shipping. There had been a wooden post or "perch" which supported a lantern on it since 1683, but this had been washed away on several occasions so a more permanent structure was sorely needed. 
The current Lighthouse rises 90 feet above the rocks, designed by John Foster and based on John Smeaton's Eddystone Lighthouse, its foundation stone was laid on the 8th of June 1827, by the then Mayor of Liverpool, Thomas Littledale. Work on it was only possible a low tides and was completed in 1830. It was built using marble rock from Anglesey and coated with "Puzzellani" a volcanic substance from Mount Etna which becomes rock hard with age. The revolving light, said to be the first in the country, last shone from it in 1973 when modern navigational radar took over.
The fort covers approximately 4,000 square yards, constructed mainly from red sandstone from the Runcorn Quarries which was floated down the Mersey and unloaded when the tide was out. The stone was soft and had to be left to be weathered. The foundation stone of the "Rock Perch Battery" was laid on the 31st of March 1826 by Peter Bourne, Mayor of Liverpool. The walls were originally 24 feet and 29 feet high, but in some cases these were increased to almost 32 feet facing the river, and had 40 foot towers. It had a slipway with three arches, also a drawbridge and a Tuscan portal bearing a Coat of Arms and the words "Fort Perch Rock". The fort, built on what was known as Black Rock, was cut off at high tides from the mainland, and stood guard at the mouth of the Mersey, with passing shipping a mere 950 yards away from the battery. The fort was armed with a total of eighteen guns, sixteen of which were 32 pounders, mounted on platforms. Single guns were placed in the towers and along the angles and two small guns faced the causeway. It had accommodation for 100 men, complete with officers' quarters and kitchen. A sunken level at the centre of the courtyard housed the Magazine and storerooms.
The Lighthouse and Fort are now "Grade Two" listed buildings.
Information gathered from http://www.merseyside.net/newbrighton/Pages/lighthouse.htm and http://www.merseyside.net/newbrighton/Pages/fort.htm