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The castle is not accessible and contains mechanical systems inside of it, it appears. The castle's forced perspective is a masterclass in design, and they did an incredible job making it appear much larger than it actually is. The bridge is only about 5ft wide for example. While it is unfortunate, the castle isn't the focus of the land and its story. The castle is a backdrop with the village as the main focus.
I believe you are right, from the concept art, there is a small entryway to visit the castle. It is such a shame that Hong Kong is not able to do that, they could've done something similar with an entryway nearby "Tick Tock Toy's & Collectibles" or somewhere near the clocktower, such a shame.
From a live video on the media day for World of Frozen, the channel host asked a cast member asking about entering the Arendelle Castle.
Arendelle Castle will not be enterable.
Walkthrough of new Frozen Land
Sneak Peak Into World of Frozen - An event that just took place inside the "Mickey and Wondrous Book" theater.
The event ends around the 54 min. mark. We got some pictures of the audio animatronics located in the Frozen Ever After ride and a short clip of a rather simple, animated figure located in the Wandering Oaken's Sliding Sleighs ride. The Play House in the Woods, a place where guests can meet Elsa and Anna which we have seen in concept art was briefly mentioned but we didn't get any details.