And here it is by day;
For a little of the history of both building and name
As you can see the tower stands on a small rock.
It is clearer in this one, that it is not very far from the town of Douglas - and that the rock is submeged when the tide is in.
Now imagine the rock without the tower, the tide as it is in that third picture, the weather as it is in the middle one - and no sheltering harbour wall.
Under those conditions, many boats and ships foundered on the Conister Rock over the years - and even though the people of Douglas could often hear their cries as the ship broke up and they were unable to get to the shore. Eventually Sir William Hilary (he of the Royal National Lifeboat Institution) who lived for some years in Douglas in a house very near where I took that third picture, decided that if a stone sheltering building stood on the rock, with the means to light a fire, and a stock of food and drinking water, in the upper room, then anyone who was shipwrecked there could wait out the storm until rescue came. He contributed most of the money himself, the rest was raised by public subscription, and in 1832 the Tower was complete.
It didn't really need to look like a small castle - but why not?
Of course, fewer ships ran aground on the rock once it was easier to see where it was - with it now having a small castle on it - but as you can see from this picture from the Wiki Commons of the sailing ship Progress, they still did at times.
I have no idea whether the crew/passengers of Progress needed to make use of the tower, or not.