The "Siege weapons" are a time limited event. The new units can only be recruited for a short period of time. The event is spread out over the course of a few weeks and the new units will be available from time to time during the event.

How many new units are there?

There are three new units. However, at the start of the event, it will not be possible to recruit all three new unit types at the same time. 

What happens to the new units after the event?

Newly recruited units will remain in your castles/fortresses/cities after the event. All of the units found in the recruitment queues will also be available upon recruitment. 

Why should I recruit the new units?

The new units feature a special ability, which makes them great conquering units. Please find more information on this in our FAQ:

Battle system & Mechanics -> Special abilities