If we consider switches in Main Building as Core switches, Distribution and Access Switches have resided in each building. As your topology shows, you have redundancy in your core layer, but you don't have it in the distribution layer (except building A). So if you want to have a stable network with a minimum level of redundancy you need to add at least 3 distribution switches in buildings B, C, D. On the other hand, the DMZ part is also needed another switch to have redundancy. Your Internet Edge router is also a single point of failure and you should add another router in order to make your communication outside, stable.