While looking for Tradescants museum I found this...

 So around the Civil wars in the 1640s (my knowledge of history is not great, sorry) London was surrounded by Forts. Lithgow walked them all (around 18 miles apparently) in one day and described them. This is a drawing by Captain John Eyre of the Quadrant Fort at Vauxhall, with possibly John Tradescants Ark behind it? This image is taken from Strange Blooms by Jennifer Potter.

Here is a map showing the position of all the forts (From http://www.fortified-places.com/london)