Welcome to the .org Celt Centurion

All of the 'barbarian' factions in RTW cannot build any of the buildings you have mentioned such as Sewers. They also cannot build stone walls even with the minor cities.

As for the Britons not getting archers that is because they have head hurlers instead which are a very good missile unit even though I have not used them that much.

If you want to know exactly what your barbarian faction can or cannot build just check the tech-tree which is located in one of the scrolls on the campaign map.

Hope this helps,
- Vip3r