Like 1830, Ur is played in three main phases. In the first phase, players can buy and sell shares in a country. In the second phase these countries build irrigation channels on the map. In the third phase water runs through these channels, this provides money for the players and the countries which can be used in the next round for further actions. At the start of the game, six minor countries are auctioned off which perform special actions and supply some extra cash for the players. During the game these minors can be sold off to major countries.
From this description it may seem like a clone of 1830 with a different theme pasted upon it, luckily it isn't.
The first difference from 1830 is the fact that a player buys shares in a country by buying an actual hex in the country. These hexes come in four flavors of increasing basic value: desert, savannah, forest and mountain. The price of a hextype decreases if one of it's type is sold and increases if certain areas of this type are irrigated. The map is very cleverly designed: three rivers flow downstream and join somewhere on the map, the cheaper hexes are located downstream and will usually be sold first. When more money becomes available, players will buy the more expensive upstream hexes. Since there is only a limited amount of water available, irrigating upstream hexes can cause the downstream countries to run dry and go broke.
When a certain amount of hexes in a country are sold then this country can start building irrigation. How things are built is decided by King, the player with a majority interest in that country. Irrigation may be built wherever the country wants, although it usually will only irrigate it's own holdings because the player gains a bonus if a hex is irrigated.
To irrigate hexes, three components are necessary: diggers, reservoirs and pumps. Diggers create the channels. Reservoirs are placed in river hexes and dictate how much water is redirected towards the channel. Pumps dictate the maximum distance water can run through the channels.
Unlike 1830, channels are not placed by laying tiles on the map; instead an erasable blue marker is used. Reservoirs and pumps are represented by a counter in the colour of the country.
These three components are bought by the countries from the bank and increase in capacity throughout the game. With the capacity increase there is big increase in price, and when a certain capacity has been reached, the older diggers will be phased out. Because every country is required to own at least one digger, it may happen that it is required to buy a very expensive new digger because the old ones have been removed from play. If a country cannot afford a new digger, the king will have to pay for it from his personal treasury.
After all companies have performed their actions the rainy season starts. In this phase water runs through the three rivers, fills up the reservoirs, runs through the channels and irrigates the hexes. There is a certain amount of water every round, represented by wooden chits. Every time a hex is irrigated, a chit is left on it. If all water from a river is used upstream then there is no water left for irrigation downstream. If no water at all reaches the end of the map, the game ends because off-map downstream kingdoms attack the players for stealing their water.
Kingdoms get money for every hex irrigated and can decide to pay this money to their landowners or keep the money for itself. If the money is kept, the kingdom has to destroy one of it's reservoirs or pumps. I found this a bit harsh: if a country expects to need a new digger soon, it cannot keep it's income without destroying part of it's essential infrastructure in the process. Proper planning will hopefully prevent this problem.
After this phase, the next round starts with a new acquisition round for the players.
Ur 1830 BC is not your average 'German game'. It cannot be taught in 15 minutes or played in 90 minutes. It is however a lot of fun, offers a lot of player interaction and plenty of chances to stab your opponents in the back. There is no luck in this game, but finding the right moment to sell your holdings may require a lot of intuition. Splotter themselves describe this game as a 1830/Roads and Boats hybrid. I mostly agree, although Ur requires lot less micro-management than R&B, which is a Good Thing in my book.