In this San Francisco 'castle', over half a dozen residents have become millionaires investing in cryptocurrencies such as bitcoin.

Dubbed the Crypto Castle, the three-story house in San Francisco's Protrero Hill neighborhood houses a revolving cast of young entrepreneurs united by their passion for bitcoin.

As the price of bitcoin has shot up nearly 600 percent of the last 12 months, to more than $4,000, many of them have minted big bucks.

The modest exterior of the Crypto Castle belies the gaggle of young millionaires who live there

The three-story house is in Protrero Hill south of downtown San Francisco. The view from the roof towards downtown is seen here

Jeremy Gardner, 25, first rented the Crypto Castle in 2015 for his startup Augur

Cryptocurrencies, of which bitcoin is one type, have shot to prominence in recent years as a radical new way to verify actions and transactions.

They rely on a public ledger, called a blockchain, that is virtually impossible to hack or forge and has implications far beyond making online payments.

Jeremy Gardner, 25, first rented the Crypto Castle in 2015 as a base for his startup Augur, which uses blockchain to run a predictions market.

The house became a landing pad for other young bitcoin entrepreneurs, who mostly sleep in bunk beds.

One, the founder of a self-driving car company that now has over $3million in funding started out living in a basement closet in the house.

Some of the Crypto Castle's young residents are seen. The house has become a landing pad for young bitcoin entreprenuers, who mostly sleep in bunk beds

The house's refrigerator is covered with stickers for bitcoin and blockchain startups

'Over a half-dozen people in the time they've lived in my house have become millionaires as a result of crypto,' Gardner told Business Insider.

Meanwhile, the fate of crytocurrencies is far from certain.

Some believe that bitcoin is in a bubble, similar to the dotcom boom of the late 90s.

For the freshly minted millionaires of the Crypto Castle, the choice now is whether to cash out while they're ahead - or double down.