Not always. Some other people could already have done the meta event before you arrived and thus opened up the entrance. In that case, you could just waltz straight in. IF however the entrance is blocked off by rocks, then yes, you'll need to do the meta event which culminates in a "hold the line" event on the bridge leading to the dungeon.
Here's a link to the event chain: http://wiki.guildwars2.com/wiki/The_Battle_for_the_Flame_Citadel_and_Senecus_Castrum
The final event is "Hold the Gates of Flame against waves of Flame Legion (70)". The other two, "Prevent the relighting of the braziers (68)" and "Protect Veragha the Shadow on her mission to kill the Flame Legion shaman (68)" are optional/not needed to open up the Citadel of Flame.