Have you completed Percival's task in Grailwood Village? He will give you the key to return to Lancelot's house (where the game started) and open the door on the ground floor to get Excalibur inside. Once you have Excalibur, you can pass through the barrier to enter the ruins.
你完成了 Grailwood 村 Percival 的任务了吗？他会给你一把钥匙，让你回到 Lancelot 的房子（游戏开始的地方），打开一楼的门，拿到里面的 Excalibur。拿到 Excalibur 后，你就可以穿过屏障进入废墟了。