Some players are looking for solid casinos. Why do they just want to play in these casinos? One of the reasons is the sign up bonus. For more information, see our article 'Why do I not receive my sign up bonus?'
So, solid casinos mean 'stand alone'-casino: they don't belong to a group. This term has nothing to do with other casino terms as single deck, single zero roulette wheel, single deck Blackjack games or single line slots. The software platform can be different: from Netent, Microgaming to RTG and TopGame. It's your choice.