Alabaster white with coal black tarred shingles on the roof, Østerlars Kirke is Bornholm’s biggest and oldest round church. It’s one of four of these intriguing structures found on the Danish island.

Østerlars Church dates to around 1150, though its roof was added much later. The building originally served as both a place of worship and a fortified refuge against seafaring invaders.

Its origins are a bit of a mystery. One theory maintains that round churches are often associated with the Knights Templar, but no one really knows for sure who built them or why. Another theory attributes the building’s shape to its possible use as an observatory.

Historic murals, which depict the birth of Christ, the gates of hell, and various foreboding Judgement Day scenarios, are a good reason to venture inside. Dating to roughly 1350, they were painted over during the Reformation like many frescos in Denmark’s oldest churches. Fortunately, these were uncovered in the 1800s and restored to their full brilliance in 1958. The church chimes are located in the neighboring bell tower.

Today, the medieval church remains a hub of modern activity. It still holds regular services, and tourists are welcome to wander through when services aren’t in session.