Many security measures are in place at Grimmauld Place: there are anti-apparation charms, the place is Unplottable, the whole house is under a Fidelius charm and it is disguised from Muggles and other interlopers. In the seventh book, it is noted that the neighbours had long ago come to terms with the houses on their square running straight from 11 to 13. It is as secure as any magical dwelling can be and can accommodate a large number of people. For this reason, it was chosen as the headquarters of the reconstituted Order of the Phoenix when Sirius offered it to the Order. Only magical people can see it, and only if told the location by the Secret Keeper himself. The house is even immune from the 'Taboo' imposed by the Ministry of Magic after the coup d'état. This is known because Hermione Granger mentions Voldemort by name at least once whilst inside the house.