The Østerlars Round Church
The largest and most impressive of Bornholm's round churches dates back to at least from 1150, and its seven weighty buttresses and upper-level shooting gallery give away its former role as a fortress. The roof was originally constructed with a flat top to serve as a battle platform, but the excessive weight this exerted on the church walls saw it eventually replaced with its present conical one. The interior is largely whitewashed, although a swath of medieval frescoes has been uncovered and restored.
You'll find a rune stone dating back to 1070 at the church entrance and a sundial above it.
A 4km cycle path to the church leads inland south from Gudhjem; the church can also be reached on either bus 1 or 9 from Gudhjem. From Rønne, catch bus 4.