At the architectural level we are concerned about:
- Services
- Components
- Security
- Infrastructure
- Backward/forward compatibility (migration, dual phase, etc)
- Scalability
- Reliability
- Federation
- Standards (not necessarily industry standards, just consistency)
Architecture is just as much about non-technical communication and business strategy as it is about anything technical. Architects are responsible for translating business goals into systems, which are implemented by developers and technicians.
As far as your existing list - "tiers" is a high-level development concept, while I would consider "roundtrips" relatively low-level. At an architectural level, and even a high-level software design level, "roundtrips" is an implementation detail of the communication between systems, and is not very interesting until it starts impacting one of the areas listed above. I'm not sure what you mean by "layering".