Most countries require you to pay for a license which is accepted through the gambling commission or to be able to provide access to other countries you will need to paid for individual licenses. For example, if you want to provide your services to the UK you have to have a license if you're operating in the UK or even in another country. There are some locations which have less restrictions than the UK, and I believe Isle of Mann is one of best options if you aim to provide a service to the UK population.
If you are allowing access to the United States this is a whole new can of worms. Basically, different states have different laws regarding gambling, and what type of license you require depends on that state. Some states are outright against gambling websites. Some countries have less restrictive licenses, but usually you will need a license which either covers multiple countries or several licenses for specific countries. There are countries which there are no licenses, however you still can't accept users from countries that require a license usually. Remember, that depending on the country you're operating in you will likely need to keep user data which means you could be subject to GDPR laws.
Opening a gambling website whether its remote or physical is a costly affair. In most countries avoiding a gambling license when it is required will land you in a lot of legal trouble.
Basically one can start operating a casino without a license, but since we are engage into business and we also look its long term success, it's better to registered it already so you won't have any changes in the future. Remember that you are carrying a name, and that name will be the face of your business, once that is ruined due to legality issues, you can easily loss your customers which you have been building for years.
You could theoretically open a casino without a license, but in the majority of countries you would be breaking the law which could result in your casino being closed, and the potential of other legal issues concerning you. You are required to apply for a gambling license in most countries before you even start earning money. Applying later when the casino has already launched, and has made an income would likely result in hefty fines, and possible rejection. When opening a gambling site its advised to talk to an attorney.
Remember, he said it is not a full casino, but some game where people compete to win money.
It doesn't matter if its not a full fledged casino or not. The gambling commission will likely treat the same. If the casino is open to the public, and advertisements have been made then it'll likely require a full license. If it was just a game between friends they usually don't care all that much.
Today one could argue if a license for BTC casino is needed at all?
If you aim to open a fully lawfully casino whether its fiat or Bitcoin, yes you will require a gambling license in the majority of countries. As I stated above, even if you opened it in a country which doesn't require a license at all you would still need a license if your casino accepts deposits or withdrawals from other countries which require a license.