Basically, areas become accessible depending on story choices. If you haven't made the respective story choices, there's nothing for you to find in these areas anyway, so it's not like you're missing out, so no need to panic. Also depending on story choices in the game (and in TW1), the guards in Loc Muinne may or may not be hostile towards you once you're inside.

If what you're referring to is the two 'entrances' to Loc Muinne, there's either via the front gate - beyond the delegation of the Order of the Flaming Rose - and another, more subtle entrance if you enter via the caves. These caves are found by going in the opposite direction of the knights. You'll find a ravine, leading to a dead end with a wrecked wagon and some corpses; on the way to this cart, almost immediately as you enter this part of the ravine, there's an entrance immediately on your right.