Start game Kingdoms, I place a garden as soon as I have 2 mats. Step 2 place a merchant on top of gardened square, allows it to build. Now mousing over square calls it a garden but demolish square calls it a merchant, either way, now all squares refuse to demolish showing the incon' label this is repeatable.

After the garden is placed the square shows empty and it doesn't matter what you put down on the square after it. The tool tip shows garden only after building something else there and all squares are non demolishable.