|(the large beach balls in the pond are a temporary anti-duck measure, to allow the water lilies to grow unnibbled)|
Technically, such a feature is called a “moon gate” (although even more technically, there should then probably be a wall for it to be a gate through).
Calling ours the Moon Gate could fit with the various astronomical themes in the garden, but we thought a bit more. We have a similar sized metal circle in the Time Garden on the other side of the house: that Time Gate allows you to pass from the present to the future. The pond-with-spaceship area forms the Space Garden, so maybe this should be the Space Gate, allowing you to pass from here to there?
But on this side of the gate is a knot garden (to be in keeping with the Tudorbethan house), which has been partially destroyed by the visiting spaceship. This area is therefore known as the Logic Garden.
And so the new gate has been dubbed … the Logic Gate, allowing you to pass from CNOT to NAND.