Hidden Side

Mystery Castle

A properly gothic castle from LEGO's boldest experiment, if you look past the dead app.

Brick Rated Score

3.7/5

Set 70437 · 2020

Pieces1,035

Minifigs6

Year2020

Set number70437

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The verdict

This one is a lovely spooky castle first and an AR gimmick second, which is lucky, because the app that ran the ghost hunting was shut down when the Hidden Side line ended.

So you are really buying a 1,035-piece gothic build with six minifigs and a couple of clever fold-open walls. If you love castles and creepy details you'll enjoy it. If you came for the AR ghosts, you missed that window by a few years.

Best for: castle fans and Halloween-scene builders who don't care about the app

The full review

What it is

The Mystery Castle is one of the last things LEGO® released under Hidden Side, and honestly it's the set that convinced me the theme deserved better than it got. Strip away the marketing and what you have is a proper gothic castle: twin towers, a domed observation tower with a telescope, spiked gates, a throne, an organ, and a strange bejeweled device tucked inside like something out of a ghost story. It's 1,035 pieces, it came with six minifigs, and it reads as genuinely atmospheric on a shelf. The oddly coloured dots scattered across the windows and spires were scan points for the companion app, which is where the whole thing gets complicated.

The catch

Here's the caveat you need before you spend a penny. Hidden Side was LEGO's big augmented-reality bet, where you pointed a phone at the model and hunted ghosts in a digital layer over the real bricks. The line was discontinued and the app was retired not long after, so the AR that this set was designed around simply doesn't work anymore. That turns the castle into a normal (if handsome) building, and it means the coloured scan points are now just slightly random splashes of colour with no purpose. On top of that, the build leans hard on two full sticker sheets for its brickwork and stained glass, and you'll find yourself rotating the entire model around between steps far more than feels reasonable. The physical play is thin too: one push-pull block at the back drops a trapdoor and flips out a couple of tentacles, and the walls fold open, but that's about the extent of the hands-on drama.

Who it's for

So who's this actually good for now? Castle people and Halloween builders. If you like a spooky centrepiece, or you collect creepy LEGO for October displays, this holds up beautifully with the app completely out of the picture. The minifigs are strong, the gothic silhouette is great, and the parts are useful if you tear it down for your own builds. It retired in December 2020 after a short six-month run, so it's a secondary-market buy these days, sitting around 170 to 200 dollars sealed against a 99.99 original price. At that premium I'd only chase it if the castle itself speaks to you. If you were hoping to relive the ghost-hunting AR experience, I'll be straight with you, that ship has sailed and no amount of building brings it back.

The parts story

What the build is actually like, and the pieces worth knowing about.

The build splits across several bags and starts a little slow, with a lot of groundwork before the shape reveals itself. Once the side sections clip onto the towers, the whole castle hinges together at the front with a clip-and-bar catch, and that's the satisfying moment where it stops being flat panels and becomes a building. There's occasional SNOT work but nothing that'll trip up the 9+ audience it's aimed at. The recurring annoyance is the rotation: you're forever turning the model to reach the next step, which breaks the rhythm on what is otherwise a relaxed, straightforward gothic build. The back is open cross-section style, so interior detail is light, but the throne, organ and that bejeweled device give it just enough story inside.

For parts fans there's real value here. Two brand-new molds debut: a dark grey Wedge Plate 4x8 with cut corners (which pairs neatly with the existing 10x10 octagonal plate to make clean octagons) and the marbled trans-blue ghost minifig lower body with hollow studs. Six elements are exclusive to this set, including trans-black windscreens that MOC builders love for spaceship canopies and dark tan corner panels. Eight more parts return after long absences, notably a dark tan brick arch that hadn't appeared in nearly ten years and tower roof pieces gone since 2013. At roughly 8.7 pence per part with plenty of big moulded panels, windscreens and roof elements in the box, the value story is solid even before you count the six minifigs.

Fun facts

01Hidden Side was LEGO's augmented-reality theme, where a companion app let you point a phone at the model and hunt ghosts in a digital world layered over the real bricks.
02The Mystery Castle retired in December 2020 after only about six months on shelves, and sealed copies now trade well above the original 99.99 price.
03The set debuted two new molds at once: a dark grey cut-corner wedge plate and the marbled translucent-blue ghost lower body used to give a minifig a floating, spectral bottom half.
04It includes a dark tan brick arch that hadn't been produced in almost a decade, making it a quiet treasure for builders chasing that specific colour.