When he knocked at the door of the gate (krousantos autou tēn thuran tou pulōnos). Genitive absolute with aorist active participle of krouō, common verb to knock or knock at. So from the outside (Luk 13:25). Pulōn here is the gateway or passageway from the door (thura) that leads to the house. In Act 12:14it is still the passageway without the use of thura (door, so for both door and passageway).
To answer (hupakousai). To listen under before opening. First aorist active infinitive of hupakouō, common verb to obey, to hearken.
A maid (paidiskē). Portress as in Joh 18:17. A diminutive of pais, a female slave (so on an ostracon of second century a.d., Deissmann, Light from the Ancient East, p. 200).
Rhoda. A rose. Women can have such beautiful names like Dorcas (Gazelle), Euodia (Sweet Aroma), Syntyche (Good Luck). Mark or Peter could tell Luke her name.