Wherefore watch ye (dio grēgoreite). Paul has concluded his defence of himself and his warning. Now he exhorts on the basis of it (dio) because of which thing. The very command of Jesus concerning the perils before his return as in Mar 13:35 (grēgoreite), the very form (late present imperative from the second perfect egrēgora of egeirō, to arouse). Stay awake.
I ceased not to admonish (ouk epausamēn nouthetōn). Participle describes Paul, I did not cease admonishing, night and day (nukta kai hēmeran, accusative of extent of time, for three years trietian, accusative of extent of time also). Nouthetōn is from noutheteō, to put sense into one. So Paul kept it up with tears (Act 20:19) if so be he could save the Ephesians from the impending perils. Forewarned is to be forearmed. Paul did his duty by them.