You set a zone of magical surveillance in a certain location. 

Choose a small building, a large tower, or an area within range that is no larger than a 200 ft. cube, in which you are present for the spell's entire casting. You place a ward over the area. The ward cannot extend into areas which you have not seen directly, including areas that you have seen indirectly, such as through the use of Divination magic.

Until the spell ends, you can concentrate on this spell for 1 minute to create an invisible sensor anywhere within the ward. You can continue to concentrate to see and hear through the sensor as if you were in its space. You can use an action to have the sensor move to any place within the warded area - the sensor cannot see anything beyond 30 ft. of the warded area. During this time, you are Deaf and Blind with regard to your own senses. A creature that can see invisible objects sees the sensor as a luminous orb about the size of your fist.

You are alerted whenever a Tiny or larger creature touches or enters the location. When you cast this spell, you can designate creatures that won't set off this alert. When alerted in this way, you immediately wake up if you are sleeping, learn the exact point where the creature touched or entered the ward, and receive a mental image of the creature as if you could see them in that moment. Upon being alerted in this way, you can use your reaction to create a sensor within 10 ft. of the triggering point - you can then concentrate on the spell as normal.

At any point for the duration of the spell, you can produce the sound of a large, ringing bell for 1 minute, which can be heard for up to 1 mile away from the location. You can choose to have the sound of the bell stop early, no action required. 

If you cast this spell again, any prior casting of this spell ends.