Just the word "observatory" as you have in your examples for Sky Tree and Umeda Sky Building would be 「展望台」.
But if you want to be more specific about the kind of building or observatory, you need to put in a little more information, because a 展望台, just like an "observatory" is not necessarily a tall building or tower. It could also be on a mountain for example.
- 高層ビルの展望台 - fits for any observatory in a tall building (like Umeda Sky Building, not a tower like Sky Tree)
- タワーの展望台 - "tower observatory"
- 山頂の展望台 - observatory on the top of a mountain
However, 展望台 are only for looking at beautiful scenery.
If you want to use "observatory" to mean "weather observatory" or astronomical observatory" then you should say