If I've build 7 districts and I build one more I've achieved the eight required - but what if I have the architect? Can I build a ninth or tenth district as well for additional points?  The rules don't say anything about building more than eight districts.

The question also comes up when playing two player games where each player controls two characters. If the first character builds the 8th district, can the second character build a ninth (or more?).

I don't see any reason why you shouldn't be able to build more. The game just end the turn eight districts are build.

There is nothing in the rules that limits you to 8 districts, so you may build more if you are able.

Building the 8th district just signals that the game will end at the end of that round; but you finish the round as normal; including building districts.

I just played my first couple of games of Citadels today (love the game) and had this exact question. And the replies make sense since the rules don't forbid going over 8 districts on the last play. Have you submitted this question to the official FFG board...?

Building more than eight districts, especially when playing with just two players, is often an excellent way of quickly turning the tables on yor opponent.  It makes that use of the Architect in this turn tempting (or judicious use of the Assassin to limit your opponent's building ability?) - it's all perfectly legal in terms of the game and provides an opportunity to whip down those last few cards to have a full colour set.

If my memory serves me right, this question was posted in a german game forum, and the answer was: yes, you can end up with more than 8 buildings.

We always play it like that.