The article mentions that the open air interior contained 3000 roller skaters wich could be seen from the exterior through the many windows. You entered the large entrance similar to a theatre entrance, went up to the first floor giving access to the 12 foot wide roller skating rink wich surrounded the open air winter skating rink on the ground floor. The illustration shows a glass wall separating the roller skating rink from the open air ice skating rink on the ground level below. Obviously there was no modern artificial ice rinks in those days. The facility also could accommodate many other activities such as curling and a circus.
The total cost of the Forum was $300,000, a small fortune in those days, the cost of an average bungalow today.