If you decide to visit Troyan from South Bulgaria in the winter you have definitely check the rode situation. The closest pass from Plovdiv,Karlovo,Smolyan is Beklemeto. Unfortunately usually it is closed in the winter. Even if it is not closed I recommend going through Shipka pass because the route is new and it is always cleaned.
Really think twice before passing Beklemeto in winter time. I have friends telling me dangerous stories about it.
Beklemeto is also known as Troyan - Karnare Pass.
As being close to Stara planina, the whole Troyan region is rather snowy in winter which makes the transportation difficult. Car-travelling without tire-chains is not recommended.