Florence Saint Lawrence's Basilica unfinished facade
Gwynedd Castles Beaumaris Castle: Work started in 1295 and continued for 35 years, with over 3,500 workmen employed at the peak of construction. Finances and material ran out when King Edward turned his attentions towards Scotland, and the castle was not completed (wiki)
Itchan Kala Kalta-minor: "short minaret": In 1855 the Khiva ruler Muhammad Amin Khan was killed and the construction of the magnificent minaret was stopped, leaving it at a height of 29m (where 70 or 100m was aimed)
Tchogha Zanbil The whole city remained unfinished after it was invaded by Ashurbanipal in 640 BC, as shown by the thousands of unused bricks left at the site. It was unlikely that many people ever lived there.