What are the necessities of a door alarm?

I'm making a door alarm, what MUST it have?

I already have; low cost, easy to use, brightly colored/visible, not too small or big (to act as a deterrent)
can you suggest some more please?

Need to give more details on what you want. What type of door? There are many to choose from, best I can guess, is that it's not a car door...
What do you want it to do when activated? Signal an alarm system, ring a bell, sirens, flashing lights, electric shock, explosion?

How do we know how much you want/need? Don't want to go overboard, and you aren't happy with what you already have. No way to gauge just what is best for you application. Is this for home, business?

Simply move away from the poor area that has thieves and murdurers to a better area that has normal people.
My neighbourhood has no crimes.

If the warm weather stays up there, I might sneak across the border...

its a simple bedroom door alarm, I'm not an electronics genius, so you can't expect much of me

it has a siren when trigered.

An alarm itself isn't a deterrent. People have to think you have an alarm. The alarm itself is optional.