[asveikau]

Old Spanish lasted until about 1500 afaik. The contrast wasn't just spelling, the spelling differences are due to deeper phonetic changes.

[anthk]

I know; but since Don Quixote and Lazarillo de Tormes the modern phonetic changes were already there.

I own an anotated Lazarillo book for modern Spanish readers, btw. It will describe you every ancient jargon and old sayings as footnotes. And, of course, some of the context.