When Bob met Alice...

Often an organization will add firewalls in an incremental fashion and end up with an administrative nightmare to manage. The firewalls are loosely arranged in a hierarchy or mesh. The answer is to connect all the firewalls to a choke as illustrated above.
The latter design ensures that no matter what the diameter of the organization, only two firewalls will be traversed. The interconnect between the firewalls, the choke, becomes a great location for a IDS/IPS. A logical hierarchy can be created using rule sets and administration is simplified as connection rulesets are limited to an ingress and egress component.
When Bob met Alice, there was a choke between them.