Istredd is glad that Wilhelm is learning from experience and he seems to be a very responsible person. Everyone makes mistakes, but he's mostly relieved that the Thorne guards didn't take that incident more seriously. At least now Wilhelm understands what kind of spells probably made his attempt impossible, so he can use them himself.

"Good job, that's exactly right."

He sounds approving because he is. The making a wall made of mental brick is what he would have recommended. Something to visualize what is going on in the mind. Istredd applies more pressure, his taps becoming more like knocks, but what he's making should hold just fine. If there is anywhere that Wilhelm hasn't quite bricked off yet, he may feel the knock harder than the places that are controlled.

"This is not unlike how to build an actual magical shield, so keep that in mind."

Which may also come in handy for Wilhelm down the road. Istredd specializes more in defensive magic and the mind because it is more natural to him. Defensive and protective over aggressive.

"What you're going to need to do is practice this regularly until it's mental muscle memory, not unlike how someone learns how to use any skill."