Located past the Gate and to the right
The commander of the Garrison, Captain Peter Saar Gwynleve, can be found here.
You come to this tower during the quest Lilac and Gooseberries to speak to the commander to see if he knows where Yennefer might be, since Gaunter O'Dim from the Tavern told you that Yennefer was spotted at this base a few nights before.
The commanding officers of the Nilfgaardian Invading Force has set up headquarters in an ruined stone tower at the garrison.
Captain Peter: How much grain will your village give?
Ealdorman: Whatever you say, your excellency
Captain Peter: Look at my hands. Look! See the calluses? These are not the hands of an "Excellency", but of a farmer. So we speak peasant to peasant. How much can you give?
Ealdorman: Forty bushels. There's be more, sir, but our lads, the Temerians, that is, took from us earlier and...
Captain Peter: You will give thirty, and that will do. Let us settle on it. And I wish to see the transport soon.
Ealdorman: Ah, thank you sir! Thank you kindly!