Was ok with āchecksā being used that way. The 3rd definition in Chambers is āto restrainā. Interesting to read up on the etymology of the expression, which seems to be that building those things in that place, until about the 15th century, was futile because they just got knocked down by the locals :-)
23d immediately reminded me of a clue from yesterday, and coincidentally that was also 23d!
Ticks for 6d, 13a and others. Good fun I thought.