The only event on the *techtree* is gunpowder. In the manual (p34), gunpowder is also the only one mentioned.

Note that there are other things wrong in the techtree :
- the mine-complex is on the level-four line, but you can build it right after you have a normal mine
- according to the techtree you need a keep to get your farmland to 40%, but in the game you don't have to
- on the techtree it puts an arrow between the keep and the church/mosque, but you also neeed the keep for the other buildings on level two (except the farmland)

The lesson is : don't look at the building-techtree too much :-)