Spanish and Castellano
These observations, while fascinating, are, I'm afraid, entirely incorrect.
What is universally known as "Spanish" is, in fact, castellano. In Spain, there are four official languages, catellano, catalán, vasco and gallego. Castellano refers to the language of the region of Castilla. In most of Spain, Spanish (AKA catellano) is spoken. In addition, in the regions where Spanish (AKA castellano) is spoken as the first (and usually only) language, the people refer to the language as "Spanish" (español). However, in the regions where Spanish is not the first language, it is considered politically incorrect to refer to Spanish as "español" since catalán, for example, is also a language of Spain and, therefore, Spanish. In these regions, it is polite to use the term "castellano" instead of "Spanish" in order to distinguish it from catalán while recognizing that catalán is also a language of Spain.
So, long story short, castellano = Spanish.
As for the lisp, I'm afraid Castillian Spanish (in Spain) contains no more lisps than the English language. The letter "z" is pronounced like the English "th" in the word "think". The letter "c" when followed by a soft vowel (i.e. "e" or "i") is also pronounced like "th". However, the letter "s" is always pronounced just like the English "s". To pronounce it otherwise (in Spain) is to pronounce it incorrectly.
Spanish and Portuguese are significantly more different than just the pronunciation of z and c. If they were ever the same, it was only as Latin.