Toompea is the hill that looms over the Old Town. It is the ideal place for a castle [which is unfortunately closed to the public] and it is where you can find the picture perfect domed Orthodox Aleksander Nevsky Cathedral as well as The Domed Church. There are yet more towers to take in, some wonderful views of Tallinn and tunnels to explore. There are several viewing platforms where you can look down upon the city below. Just look out for the signposts.
Aleksander Nevsky Cathedral
Built in the same style as the 17th Century domed churches found in Moscow, this late 19th Century cathedral was built by the Russians as a symbol of their power. So it is a copy of sorts, but a very good one.
Kiek in de Kok Tower and Tunnels
This ginormous beast of a tower was rebuilt in the early 16th Century replacing a smaller tower. It now houses an interesting museum that covers the turbulent history and development of Tallinn up until the 18th Century.
It is also the entrance to The Bastion Tunnels which were dug in the 16th Century as a secret way for soldiers to get from A to B if they were under siege. They were utilised again as a bomb shelter in the Second World War and more recently as an adopted home for those less fortunate. There are guided tours of the tunnels.