What is CTR – Control Zone?

A control zone is a block of Controlled Airspace extending from the surface of the earth to a specified upper limit. (ICAO Annex 11 — Air Traffic Services)

Control Zone (CTR) is an airspace designated by the authorities for the purpose of providing ATC service to aircraft operating within this designated airspace. CTRs are typically established around airports to provide separation and orderly flow of air traffic. Within a CTR, air traffic controllers are responsible for issuing instructions and clearances to aircraft to ensure safe and efficient operation.

The lateral and vertical extent of control zones is detailed in the appropriate national AIP (Aeronautical Information Publication).

Airport Control Tower - ATC Tower
Airport Traffic Control Tower and CTR (Control Zone)

source: ICAO Annex 11 — Air Traffic Services