You're right. My mistake. It must have been in the courtroom or somewhere else.

 

My theory:

-Fudge, in his great wisdom, decided to weaken the Ministry's defences for some random reason like budget cuts. Maybe Lucius was able to bribe him to do it or something like that. After all, he's still denying that Voldemort has come back.

-This goes with several other theories mentioned before. Maybe there were a lot of defences, but Voldemort or the death eaters were able to take them down. He could've done a ritual, for example. I vaguely remember Harry being worried at the lack of defences in the DoM during the book. This could also give a reason on how his group of 6 untrained children were able to fight 12 (I think) death eaters off for a while; the death eaters were weakened by the wards.