We usually have a lot of layers in a single drawing, is there a way to merge certain drawings into one?

Yes, a simple LAYTRANS command will do!

* The first step is to execute the LAYTRANS command, more about LAYTRANS command

the Lay Translator dialog box would pop up, as shown in the following picture,

 * then click “New” button to open the “New Layer” dialog box and specify the Name, Linetype, Lineweight, color and so on, here I name the new layer as “room” and click “OK” to confirm,

* Select the newly created layer “room”, then select the layers you want to merge from the left “Translate From”, then click “Map”,

We can see from the above picture that the layers we have selected are displayed, then we can click “Translate” to complete the merging