"viewing tower" is a correct and usable phrase in written English.
It can be used to refer to a structure built for the purpose of viewing something. For example, "The visitors climbed to the top of the viewing tower and saw a breathtaking panoramic view of the city."
Telecommunications Tower (Barcelona, 1992) Impressive concrete masts complete an awe-inspiring viewing tower.
The sprawling park has a tree-top walkway (22), viewing tower (23), playground, arts and crafts shop, restaurant and café.
From a viewing tower you can see the beautiful mangrove-fringed lagoon with its resident colony of flamingos.
Photograph by Ian Teh A viewing tower on the frozen banks of Ngoring Lake rises above China's highest wetland ecosystem.
The 760-acre state park is off limits to cars, but trams ferry visitors to various trailheads, bird blinds and sites like the two-story hawk viewing tower.
The British Airways i360, a viewing tower that stretches up to the clouds, stands right next to the ruins of the West Pier.
The surrounding landscape is a snow globe of rocky terraces, skyscrapers, viewing towers, garages and tree-covered knolls.
The elusive purple hairstreak butterfly can be seen from the top of one of the viewing towers overlooking oak woodland.
Critics say that will be lost if the development – which could include a restaurant, day spa, apartments and two metal viewing towers – goes ahead.
Future projects are likely to be favoured by installing viewing towers at the tops of turbines to encourage visitors and in selling shares in the schemes to local people.
South of the castle were informal gardens and orchards, with the bases of 16th-century viewing towers remaining at the corners of this drystone-walled enclosure.
