Are ATC towers built in warzones?
Yes, sometimes control towers or other ATC facilities are constructed to support air traffic control. If, for example an ATC tower is needed (there was none previously or it was destroyed), a mobile replacement is used.



For mission control Airborne Warning and Control Systems (AWACS) are used:



But these are quite expensive to fly. If possible, mobile surface based radars are brought in to provide a surveillance picture. 

Mobile towers, radars and navigation beacon are not solely used in war situations. In natural disasters they have proven to be useful as well: 

FAA sends temporary Air Traffic Control tower to Haiti