From Tsethang downtown take bus #2 for Y1 RMB on the Nedong street in south direction. After about 20 minutes you will get to the last stop of the bus which is on the parking lot under the Yumbulakhar. Bus goes every about 15 minutes in both directions. From parking lot you can rent a donkey, horse or camel to get to the gate.
Walking by stairs it takes up to 30 minutes to walk up to the gate. Enterance fee inside of Yumbulakhar is Y30 RMB by winter 2015. Nowadays castle is a small monastery, inside you will find many statues of Buddha etc. Don't forget to walk up the stairs next to the castle to enjoy magnificent view of the Yumbulakhar with many colourful praying flags and whole valley.
On the way to Yumbulakhar by bus you will pass around many interesting places in Tibetan history. Tibetans believe in mountains around Tsethang lived monkeys who later became humans. Also first Tibetan kings came down from the sky in these mountains. Bus also stops in front of Thradrug (ཁྲ་འབྲུག་) monastery. This is one of the first monastery in Tibet built in the end of 8th century.
Around parking lot is many small restaurants, shops and hotels.