In economics, a monopoly is a situation where there is only one provider of a product or service. In EU2, it is a quite different beast. An EU2 monopoly is a situation where one country has successfully placed 6 merchants into a particular center of trade.
How to get a Monopoly
Getting a monopoly works using the same mechanism as sending traders. You select the CoT, and click "send merchant". If you had 5 traders already in place, and there no current monopolist in the CoT, and there are 19 traders or less, and your placement succeeds, then you get the monopoly. Your shield moves up to the top slot (6 merchants placed).
A country must have trade tech 3 or higher to be allowed to attempt to place a merchant in a CoT if it already has 5 in place. Below tech 3, any attempt to place a merchant when there are 5 already in place will fail.
Effects of Monopolies
The country which has gained a monopoly gets several benefits from it:
- it gets an extra +1 merchants per year
- its trade efficiency is increased by 2%
- it gets the trade income from any empty slots in the monopolized CoT
When other countries attempt to place merchants in a CoT with a monopoly, they have a much easier time displacing a monopolist's 6th trader than they do his other traders.
Getting monopolies in any non-competitive CoTs is basically a no-brainer. You pay once for the merchants, then just forget about them and reap the benefits.
In a competitive CoT, it is very difficult to hold a monopoly for long, even with a huge advantage in trading efficiency. On the other hand, the 2% boost in TE for a monopoly can make having one for even a few months pay off. Consider a fairly normal game around 1600: world trade by this time may be 10000 ducats. A human player concentrating on tech, and using good empty slot technique, should have 5 merchants in most CoTs by this time, and a few easy monopolies in colonial CoTs nobody other than the owner knows about. So he'll control roughly 30% of world trade -- 3000d per year -- which is modified only by his trading efficiency. Boosting TE by 2% thus pays 60d per year, or 5d per month. Thus, getting a monopoly even for one month is worth doing if the merchant placement cost is 5d or less.