Ask a locksmith which single upgrade is worth the money on an ordinary front door and most will say the same thing: the cylinder, if yours is exposed.
What snapping actually is
The euro cylinder is the flat, figure-eight barrel used on the great majority of doors fitted in Spain in the last few decades. It is held in place by one screw through the edge of the door, and its weak point is that it is made of brass, a soft metal, with a narrow waist in the middle where the screw passes through.
If the cylinder projects beyond the handle plate, it can be gripped and snapped at that waist. On a standard cylinder the break exposes the internal cam, which then operates the lock without a key. It is quiet, quick, and leaves almost no other trace, which is why people come home to a house that has been entered without a single broken window.
How an anti-snap cylinder answers it
An anti-snap cylinder is designed to break in a controlled way. It has deliberate sacrificial lines cut into it, so that when force is applied, the outer section shears off cleanly and leaves the working mechanism intact and protected inside the door.
The attacker is left holding a piece of brass, with a lock that still does not open. That is the whole idea: not a cylinder that cannot be broken, but one that breaks in a way that helps you rather than them.
Good cylinders combine that with other measures: hardened pins that resist drilling, and anti-pick features inside the mechanism.
How to tell what you have
Two checks, and both take a minute.
Measure the projection. Look at the cylinder from outside. If it sticks out more than two or three millimetres past the handle plate, there is something to grip. Flush is what you want.
Look for markings. Security cylinders are usually stamped or laser-marked on the face or the cam with the standard they are tested to. A cylinder with nothing on it at all is almost certainly a basic one.
If you cannot tell, a photograph of the cylinder face and of the door edge with the door open is enough for a locksmith to identify it.
Does your door need one
It depends on how exposed the door is, which varies enormously across Altea.
Worth doing: a street-level front door, a door on a shared landing, a villa entrance reachable from a public path, any door on a property that stands empty for months.
Less urgent: a flat door behind a locked communal entrance with a concierge, an internal door, a door behind a locked gate in a supervised building.
What it does not fix
An anti-snap cylinder protects against attacks on the cylinder. It does nothing about a door that can be levered, a frame with short screws, a strike plate fixed only into the architrave, or a terrace door left open because the mechanism no longer locks.
Security is the whole door. Fitting an expensive barrel into a soft frame moves the weak point rather than removing it, and a good locksmith will tell you that rather than selling you the cylinder alone. That is the difference between an upgrade and a purchase.
If a break-in has already happened, the repair and the upgrade are one conversation, and the insurer will want the details of both.