Tiny Turrets

These tiny mud turrets appeared in the solid clay pathway outside our window.
They are about 30mm high and 15mm across the circular top.

They have been built of little balls of mud with some gaps in between.

A matchbox perhaps shows the size quite well.

Although the small river pebbles are clear around the towers,
I can't tell whether they've been specially cleared for the purpose.

I've tried shining a torch inside to see if there's a hole to an underground passage, but can't really tell.

There were three turrets, but one has either crumbled or been broken.
Although I've spent more time than I should have watching for 'the builders',
I've seen neither ants nor wasps.
I'd really like to know who has built these small adobe towers.
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