How does the grammar behind "though not so soundly but that" work?

Question

I was recently regarding A Child's History of England by Charles Dickens when I came across this sentence:

However, for once that the bold Britons beat him [Julius Caesar], he beat them twice; though not so soundly but that he was very glad to accept their proposals of peace, and go away.

I don't quite understand the bolded part. Did he beat them soundly? Not so soundly? Was he glad to accept their proposals of peace or not? How does the grammar function here?

Answer 1

The archaic bit is the use of but.

I believe the passage means "not so soundly that he wasn't prepared to accept...".

In other words, if he had beaten them even more soundly, he wouldn't have gone away, but would have stayed to rule; but they put up quite a fight (so he couldn't beat them as soundly as he might have wished), so he accepted their proposals of peace.

It's not at all clear to a modern (native English speaking) reader.