Futsal is the official version of indoor soccer. Wall soccer will soon lose popularity and is already losing participation in the Midwest and East coast. There are so many advantages than wall soccer and F.I.F.A. agrees as it is the only sanctioned form of indoor soccer.
An outdoor soccer field is 5-6 courts. In any one of those courts there is a 3-5 attacking and defensive players. Playing this 5 aside form of soccer puts you in any one of those sectors increasing exposure to small space, passing, moving without the ball, quick thinking, and shooting with intention versus opportunity. This form of soccer is actually more of a workout because you are constantly moving moving from defensive to offensive situations in a blink of an eye. There is no time to walk and take a breather. With only four field players you are truly engaged the whole time 100%.
The rules are quite simple. you play with 4 field players and one goalie. There are no throw-ins. The player places the ball on sideline and has 4 seconds to kick ball in too a teammate (it is an indirect kick so no trying to make a goal from sideline).
Even though you may not have a court or low bounce ball to play true futsal replicate the rules on natural grass. you will still get the benefits reducing field size and mimicking the rules. This form of play will greatly enhance awareness and speedy decision making.