A constitutional monarchy is a parliamentary democracy in which the non-executive head of state is hereditary instead of elected. A parliamentary democracy is a system in which an elected assembly, a parliament, is sovereign (forming the executive itself). This is distinct from presidential system, in which executive power is given to a directly-elected head of state, while the assembly is restricted to legislative functions.
A parliamentary system can be more or less democratic than a presidential one, depending on the electoral system(s). At one extreme are the British and American systems, and I don´t know which is really less democratic. Both effectively limit political power to two parties by using a first-past-the-post system which ignores geographically dispersed votes: a 1% share of the overall vote that´s concentrated in a single constituency is more valuable than 10% spread evenly among all. The result is that artificial majorities are created (e.g. most post-1945 British governments have been formed on the basis of 40-45% of the vote for the governing party).
The other extreme is a party-list system, as was used in the Weimar republic (I don´t know off-hand where it´s used now). In this system, parliamentary seats are distributed on the basis of the percentage of the vote won by each party, so a 10% share is ten times as good as 1%, no matter what. This arguably gives too much power to the parties, however, since it´s the parties that create the lists. The result is there´s almost no way to get rid of high-ranking party officials (who get the best places on the list, of course!). The other important weakness is the lack of a link between each elector and a member of parliament; the vote is only for a party and not for any particular person.
Most systems try to find a middle-ground, in various ways. The Imperial German system was similar to the British one, but with a second round to ensure an absolute majority for every winning candidate. France currenty uses such a system, though within a semi-presidential one. The post-1949 German solution was to provide two votes: one for an individual member, and another for a party, with some restrictions to keep out fringe parties. This prevents the problem of artificial majorities, while trying to duplicate the benefits of the link between each elector and a member of parliament. It also makes it less impossible to remove individuals high on the list, since directly-elected members take precedence. In practice, however, that problem exists, and even the personal vote tends to be very party-based (but this is also true with the personal votes under the British and American systems).
There are a lot of other systems too, each with strengths and weaknesses. The structure of the executive (parliamentary or presidential) doesn´t really matter so much, but the structure of a second house/chamber can be very important, especially in federal systems where it safeguards the rights of individual states.
Hmm, now that I´ve finished my rant, I think constitutional monarchy would be a good addition.
Maybe you could add the Magna Charta as a wonder.
The trouble for me is I´d want to use democratic socialism *and* constitutional monarchy. Economic and political systems really aren´t the same thing except in totalitarian systems like Marxist socialism or fascism.
