Casinos look a little more secure then they actually are. The big hole in security now, and as well as in the age you are looking at is that they need large amounts of cash at hand. That's cash that is at the cashier and at the table games they might play. There is cash that is coming in constantly from the players, and cash to pay players off that simply needs to be easily accessible.
One can go in a modern casino today, and at the main cashier there are millions in funds, that sit in a drawer unlocked if the cashier is standing there. There is no particular security that will stop a determined thief. There are no armed guards, the clerk is going to hand over the money rather then risk their safety.
Back in the era concerning your question, the setup would not be much different. There would be a safe, but it would be in a back room. The clerks station, called a cage today would still be rather exposed and set up to move money with very little if any security that would stop a determined thief. The cage would be far away from an entrance as possible. This is true now and would be true then.
There would not be anything to fancy in the way of security except bars on the cashiers window and ugly tough looking security guards. The main bit of security is that then and in the not to distant past, gambling houses were always owned and operated by people that were tough, mean, ruthless and had ways to get to anybody that robbed them, that conventional law enforcement did not have. In other words they were gangster types no matter what era they were in. They were well connected in the under world of things. It is quite likely that anyone robbing a casino was making a bad bet with their life, and this was the strongest deterrent to crime against a casino.
For your story perhaps the actual robbery is not to complicated, getting away with the heist and staying alive long enough to enjoy the money is the main problem for the thieves.
Some after thoughts:
In the time frame your considering, in the western world at least, Casinos did not look like anything that one might imagine as a casino or gambling house. Table games like black jack, roulette and other games where one makes wagers against the house, and a employee is used to run the game, did not start developing until one or two hundred years after the tech time frame you are considering. The games before that would of been a poker type card game, were they are betting on some card game between the players, in the back room of the tavern. It might be behind a closed door. Your bandits will essentially be robbing the players and not the casino. The players would be playing with coin of the realm or gold rather then some type of casino chips, so most of the money to be had would be at the game and on the table.
The games going where more of a private affair. One could not just walk in and sit down as you can do at a poker table or twenty one game today. The villains in your plot would need to connect to some degree with local gamblers to find out about the game, when it was played and about how much was being played for.
Most gambling, especially at higher stakes was done in very private spaces, and the type of gambling in more public places like a bar that might offer some gaming was of a lower limit more sleazy type of character. The rich games were parlor games in the dukes castle or a rich business mans mansion.