Complex sentence: A large wall, 75 meters on a side and 5.5 meters high was planned to surround Uraniborg, but never built, instead a high earth mound was constructed and lasted until today being the only remnant of the observatory still in place.
Split into simpler sentences: A large wall, 75 meters on a side and 5.5 meters high, was planned to surround Uraniborg, but was never built; instead, a high earth mound was constructed. That mound has lasted until modern times, being the only remnant of the observatory still in place.