void OnStart()
{
SetEntityPlayerInteractCallback("CastleGate", "StartMap", false);
}

///CHANGEMAP
void StartMap(string &in asEntity)
{
if (HasItem("lantern") == true)
{
ChangeMap("01CastleBasement.map", "PlayerStartArea_1", "", "");
}
else
{
SetMessage("Messages", "FindItems", 4.0f);
}
}

When player interacts with "CastleGate" it then start the function StartMap. IF the player have the item "lantern" = true, then it changes map... ELSE it displays a message on screen telling you to find the items.

thats how it works in my case, but you can just switch the script around to suit what you had in mind.