Learning to spot weaknesses in enterprise network diagram

I have a network diagram but I have an issue translating certain characteristics of its topology.

For example I have an edge router, a firewall, then another router which connects with 3 others (these are used for vlan communication ). I know that the router right after the firewall propably causes a weakness but I don't know what.

Also I would like to ask if two vlans are connected to the same router via switches (one for each) are possing a security issue.

Are there any resources available for studying and learning about such topics? Or am I just failing to combine the things I already know about networks?