I'm not familiar with that particular brand of guitar.

I did some googling and found some information about Castilla guitars, but not enough to pin down the guitar in question because they apparently make a number of different models, including both steel-string guitars and nylon-string classics. And solid-body electrics.

Maybe some of this will help identify what type it is. These are close-up photos of a typical nylon-string classic guitar (courtesy of Elderly Instruments).

CLICKY.

Note particularly—the close-up photos of the headstock. Classics have a slotted headstock, although some steel-string guitars do also. The give-away are the parts the string winds around. On a nylon-string classic, the barrels are fairly large and are generally made of ivory, bone, or plastic. On a steel-string guitar, they are not quite as "fat" and are made of metal.

Then, look at the close-up photo of the bridge. This is a dead giveaway. On a classic, the nylon strings are tied to the bridge rather than slipped through holes in the bridge and held in place with bridge pins. It the guitar has this kind of tie-on bridge, then the guitar should absolutely not be strung with steel strings.

Some people are under the impression that the three bass strings on a classic are steel because they look like steel strings. But they actually have a nylon fiber core and are wrapped with fine silver (or "silver-ish") wire. The three treble strings are obviously nylon. On a steel-string guitar, the four lowest strings are wrapped (with a steel wire core) and the two treble strings are obviously bare steel wire.

If this is a nylon-string classic and it has steel strings on it, they should be replaced immediately, least the higher tension steel strings (two to two-and-a-half times the tension of nylon strings) warp the neck and/or soundboard or start pulling the guitar apart. A classic is not braced to take that kind of stress, whereas a guitar built for steel strings is more strongly braced.

Don Firth