The 86ft (26m) high Naze Tower at Walton-on-the-Naze, Essex, was covered in scaffolding in November.
Work was undertaken to alleviate damp problems, which involved replacing cement with lime mortar to allow the building to breathe by allowing air to get through.
The Tower has also had internal walls lime washed and cracks repaired.
The tower, which overlooks the North Sea, Hamford Water and the ports of Felixstowe and Harwich, was originally built as a shipping aid and would have had beacons lit on it when it was built in 1720.
It was taken over by the RAF in World War Two and used as a radar station.
The Nye-Browne family bought it in 1995 and it is now used as a visitor attraction.
Owner Michelle Nye-Browne said: "It's really about making sure we save it for future generations and it's in good condition so that it's a useable building.
"As the repairs were being carried out we discovered embedded steelwork was rusting and it, the tower, was destroying itself."
About £170,000 of the funding was provided by Historic England.
Claire Fidler, heritage-at-risk project officer for Historic England said: "We are pleased we were able to step in and offer timely advice and grant aid, ensuring it survives for another 100 years in its unique spot on the cliff edge."
A grade-II listed coastal tower is re-opening after a £250,000 campaign to save it from "destroying itself".