According to the Portsmouth Museums website (http://www.portsmouthmuseums.co.uk/museum-service/The-Round-Tower) the foundations of the Round Tower date from 1418, Queen Elizabeth I ordered it rebuilt with more cannons in mind and it was heightened during the Napoleonic War. In the latter years of its defensive career it formed part of the Point Battery and Barrack, developed in the 1850’s, and the extensive use of red brick around the Tower gives the area an overwhelmingly Victorian feel, despite the Towers much older pedigree.
Why is this important? It’s important because the spot, Tower or no Tower, affords excellent views of the harbour. Over the years the Tower has been rebuilt and adapted to allow for even greater view of harbour until the Coastal Artillery was disbanded in the 1960’s. Now it is owned by the Council and you stand on top of it, taking in the view, all year round. I noticed, whilst I was up there, that the Isle of Wight ferries pass very close to the Tower and the fortification also overlooks a small beach, which runs from the base of the Round Tower and its nearest neighbour the Square Tower.
The Portsmouth Guide (http://www.portsmouth-guide.co.uk/local/sqrntwr.htm) has more more facts about the Round Tower and its partner the Square Tower and I have already mentioned the link to the Portsmouth Museums Portal.