The Monument:
Of course it's not an Aztec temple or other ancient place of worship but it looks like one (especially from a height - just look at Google Maps). This structure is known as the Wellington Monument or Wellington Testimonial. It was built to commemorate the victories of Arthur Wellesley, 1st Duke of Wellington who was born in Dublin. Known as "Iron Duke", he was a famous an Anglo-Irish soldier and statesman, one of the leading military and political figures of the 19th century, and the victor of the Battle of Waterloo.
This monument was completed in 1861 and it's the tallest obelisk in Europe at 62 m high. It was intended to be taller but funding ran short.
The cache:
The cache is a small size container and holds just a logbook and pencil. Good luck!
