The best way to secure two or more beams together is to use a carriage bolt that passes through all the pieces of wood and is fastened at the end with some type of locking means, such as a lock washer and nut. This is through-bolt construction. This method provides secure, reliable joints and allows the bolts to be adjusted as many times as necessary without weakening the structural integrity of the system. Through-bolt construction also allows the system to be disassembled or reassembled without damage. Good news if you decide to move and take your play system with you.