There are 14 National Wonders. The series will consists of the following four parts:
I. Click here for National Wonder #1: Flop 3
II. Top 3
III. Good. Basic. Not Game Breaking.
IV. Specific. Key for some scenarios.
PalaceII. Top 3
III. Good. Basic. Not Game Breaking.
IV. Specific. Key for some scenarios.
Cost: 107
(quick speed, 160 normal speed)
Effects: +1
, +8
, +2
, +4 
Requires at least 6 cities (depending on map size)
The Palace marks a city as the capital of your empire. It comes for free, though can be build in order to relocate the cap to another city. It is without a doubt the strongest building in the game. That is due to its ability to mark the target for Bureaucracy, due to its timing (it´s there from the start for free!) and simply it´s abilities:
Effects: +1
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Requires at least 6 cities (depending on map size)
The +2
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The +4
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The +1
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Most importantly there is the +8
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Without the Palace for free in your first city, nothing would work. The game would look entirely different, most importantly due to the fact that tech costs in relation to the accumulated research would become extremely high. It wouldn´t be possible to pay the upkeep for more than a few units and in general the game would become very, very passive until a significant amount of cottages started kicking in.
University of Oxford
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Cost: 268 (Double Production Speed w/ Stone)
Effects: +100%
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Can turn 3 citizens into Scientist
Requires at least 6 Universities (depending on map size)
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The boost from Oxford (together with it´s prerequisite, the Universities) marks a significant point in an Ironman or ffa game. Working towards that point and making sure the Universities and Oxford are finished as fast as possible is a very important aspect of such games. Look out for a possible stone you could still hook or plant in order to get double production speed for it.
In all kinds of OCC games, the importance of Oxford cannot be stressed enough and gaining only a few turns on your opponent when constructing it can result in a significant advantage in research. No matter whether it´s an ancient or renaissance start, Oxford is key here.
National Epic
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Cost: 167
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Effects: +1
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Can only be built on Renaissance and earlier starts
Requires Literature and a Library
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In OCCit´s "do or die", in Ironman there are times where you don´t build it, but that isn´t necessarily a good omen for your end result. It´s not a must build like Oxford in those kinds of games, but it comes close.
Click here for National Wonder #1: Flop 3