Before rP22984 siege units were produced in the fortress and you could house up to 20 siege units there.
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Since then, any type of siege unit can only be produced in the workshop. For such a large footprint, it can only garrisson two siege weapons at the time, this patch aims to increase it to five.
The elephant stable aslo allows for 5.
PS: The garrison limit was set to two in rP20457, but without an explanation.