Grimmauld Place "every security measure known to wizard kind"

[Mark D.]

No: HPFGUIDX 72132

On page 115 of Ch 6, Sirius claimed his father put "every security 
measure known to wizardkind" Also "and now Dumbledore's added his 
protection" It unplottable and protected by the the Fidelius charm, 
but is this protection foolproof?

Agreed, Grimmauld Place is impressive, except WHY are the Weasley 
twins able to apparate inside it while iin Hogwart's no one can 
apparate?

Also several darkwizards (those of Black ancestry) knew of Sirius' 
house on Grimmauld Place. Could they go there from their memory? 
Also what about the floo network?

Dumbledore admitted he wasn't infallible in the OOP ending. Do you 
think that security should br doublechecked?