This watchtower (ta dzong) overlooking the dzong now houses an excellent museum. The five floors of displays tell the history of the monarchy through such varied treasures as the 500-year-old jacket of Ngagi Wangchuk, the second king's saddle and a copy of the famous raven crown. You can drive here and then walk back to town down a staircase past several chapels.
As you ascend the floors look for the radio given by American Burt Todd, a friend of the third king who visited Bhutan in the 1950s (the first American to do so) and set up Bhutan's postal system. Other treasures include a fine photo of the kings of Bhutan and Sikkim in Kolkata (Calcutta) in 1905, and a beautiful 1926 document declaring an oath of allegiance to the monarchy.
The most sacred religious item is a copy of the Padma Kathang, a copy of Guru Rinpoche's biography discovered by Pema Lingpa underneath the Jokhang Temple in Lhasa. There are two lhakhangs inside the ta dzong; the top-floor Gesar Lhakhang is dedicated to the 19th-century penlop of Trongsa, Jigme Namgyal. Two British soldiers are said to have been kept in the dzong's dungeon for several months during the Duar War.
There are sweeping views from the roof, plus a souvenir shop and a ground-floor cafe providing refreshments (and lunch by prior arrangement).
Photos are not allowed in the museum, so leave your camera and phone in your vehicle. There are lockers at the entrance but you'll need to return all the way back there to pick them up.