Minimal gun locker requirements.

Firearms should be stored in a safe and responsible fashion. Being small, lightweight and expensive, they are great pickings for criminals bent on quick cash. Guns of less monetary worth could be used for other crimes, or even turned on you should an intruder lay hands on them.

Another peril is a child coming across a firearm. I'm well aware that children can be taught to not touch guns, as I learned these lessons from both my birth father and my stepfather. Aged seven I knew not to touch my fathers target pistol, which he never needed lock up from me. In retrospect, he probably kept it unloaded at all times when not out at the range.

Still, no matter how well you have taught your children, they should never be allowed to handle firearms without your knowing, preferably under you supervision. If your child ever brings a friend over, there could be a child without even the basest firearm safety lessons learned under your roof.

You probably know that concealment doesn't equal cover, the same applies to your firearms. Just because they're not in plain sight, or even difficult to reach for younglings, doesn't mean he/she/they won't come across them. in the long run, your kids will be peeking at and into every closet and corner of your house. Guns shouldn't be in unlocked cupboards, or in a box on top of a tall closet, they should only be accessible to you.

Trigger locks are not a viable option. Guns equipped with these locks can still be stolen, and in some cases even be fired with the lock in place! A gun equipped with one of these is in no way to be considered safe. Safe gun locks are available though, they usually block the chamber, running from the breech through the barrel, out the muzzle. Other locks may pass through magwell and ejection port.

A decent safe or locker maybe cheaper than buying locks for your entire collection, and even guns equipped with state of the art locks can still be stolen and forcibly unlocked at another location.
In come locked doors. Ideally, you could have a true gunsafe, which is secure both from children, burglars, fire and even moisture thanks to a built-in desiccant compartment. Many safes are very heavy, making it impractical fro a burglar to run off with the entire thing.
Almost as good are sturdy closets, dressers, (foot)lockers or cupboards which can be properly locked.
Just the lock will probably deter most "in it for the quick pickings" burglars, but not one who has already established that you're not home, and whose curiosity has been spiked by a locked door. Criminals know that a locked door inside a house frequently stands between them and quick cash. chances are they'll pass on breaking it open if they're busy hauling out a TV, X-box and an assortment of jewelery, but if they can pick up the locked box and take it with them, they just might. Bolting the locker to a brick or concrete wall is an option.
If you'd still like to be able to move the locker around, you can bolt a ring into the wall and make a small cut into the back of the locker so you can attach it to the ring with a padlock on the INSIDE of your makeshift safe. You shouldn't be able to move the safe around if it's locked against the wall. Easier than a ring would be an L-shaped piece of metal, one end is bolted to the wall with a screw and washer, the other part which points away from the wall can slide through a small slit into the locker, where a padlock can be attached.

The locker should not be easy to disassemble. A lock is no good if all you need to remove the front door is a screwdriver in order to pass it by. Other weak points are the hinges. They should be welded onto the inside of the locker. If they're on the outside, they could be smashed or cut, if they're screwed, the screw could be drilled.
The locks on the front too, should be protected by the metal plating. Padlocks Are the worst of all, they can be cute in a matter of seconds with a pair of bolt cutters, drilled with a portable drill, or circumvented by cutting the piece connecting it to the door.
A lock which protrudes outside of the box can have it's cover removed with minimal force, and might be prone to being turned with a pair of pliers.

A steel/aluminium dresser which has been welded together and bolted to a wall, has no external hinges and a lock which does not protrude from the door makes a great gunsafe, which keeps your guns safe from all but the most determined criminals, and keeps your children and their peers safe as well.
Another option is to convert a walk in closet to a walk in gunsafe by equipping it with a hard to break door, deadbolt and internal hinges. Essentially the same, only bigger.

Next up, home workshop passive Desiccant containers.

Thank you Zonk, for pointing out some typo's