It's just as funny the second time that Maomao thinks "yeah renshi lied about a ton of stuff but he couldn't possibly be lying about his age. well. that's enough of that thought."
Officially it's a castle (城) with castle walls, but it's more fitting to call it a fortress (砦) because the structure is more defensive, hence more like a fortress, than a castle is--is how I interpreted that line. I suppose some rewording for that line is in order.