It all depends on who the 'jumpers' will be and how much suitable space you have. Don't fall into the trap of choosing by the images on the front. In our experience too much emphasis is placed on the castle matching the 'theme' of the party when at the end of the day the kids mostly comment on the fun they had inside it! A product that looks in theme could be a poor choice when it comes to suitable features, size or power requirements.
Match the 'jumpers' to the features of the castle such as whether they would enjoy a slide, ball hoop and ball, an internal soaker hose for wet play or a large jumping space for games and free-play. Special needs or nervous children can benefit from an all-ages castle that can allow carers, parents or educators go inside with them. A party or public festival attracting both girls and boys should choose a uni-sex style that makes everyone feel comfortable.
The size of the castle is not the same as the size of the space required. As a general rule 1.5m all the way around is recommended. so a 4m x 4m castle would best suit a 7m x 7m space.
A good supplier will provide details on area and power requirements for all castles you are considering as well as personal advice on which castle would best suit the age group and needs of the 'jumpers'. A simple 'choose by theme' approach should be avoided; instead select a local supplier who provides the extra help and information to make sure you choose the perfect castle or other inflatable for a successful event!