Usually, the fastest and most convenient way to reach Buzludzha (Бузлъуджа) is by car. But not always. Not necessarily.  And still sometimes the only one.
A possible combo is to take a train to the small town of Kazanlak (Казанлък) which is located approximately 20 km away from the monument. From Kazanlak you can take a 10km taxi ride to the foothills of the mountain from where you can either hike or hitchhike up to the peak of Buzludzha and its infamous monument.

During winter times with heavy snow hiking up the last 10 or so kilometres is the only possible way to reach the monument, cause in this part of the world, they don’t clean

up mountain roads leading to abandoned monuments .

Generally, you should know that you are on your own. Be prepared to face extreme weather conditions year round, while winter time can be especially severe.
Strong winds are typical . The weather can change dramatically in a matter of minutes. Dense fog falls quickly, so when in such conditions its crucial to stick to the main road – going up or down the short cuts can get you lost easily.

Be sure to have appropriate foot gear and clothing. Also carry some food and water supplies in your backpack, cause up there is no McDonnalds  (yet).

There are two huts with same name, Buzludzha that offer simple accommodation and food. There is also a sleek renovated hotel, which seems to be always closed.  Usually not full but weekends with good weather can be crowded. Wild camping is an alternative, if you know what you are doing and are not too obvious.

Yes, I know that you want to sneak inside just for few minutes to take a photo of that notorious hammer and sickle, but the good old urban times of Buzludha seem over now.
The monument is guarded by police 24/7 and there are surveillance cameras and motion sensors inside.
While you would need special batman skills to get inside, enjoying the decaying structure from outside is still free and worth it.