Passage: Some historians regard the period from 1621 to 1625 as a distinct portion of the Thirty Years' War, calling it the "Palatinate phase". With the catastrophic defeat of the Protestant army at White Mountain and the departure of the prince of Transylvania, greater Bohemia was pacified. However, the war in the Palatinate continued: Famous mercenary leaders – such as, particularly, Count Ernst von Mansfeld – helped Frederick V to defend his countries, the Upper and the Rhine Palatinate. This phase of the war consisted of much smaller battles, mostly sieges conducted by the Imperial and the Spanish armies. Mannheim and Heidelberg fell in 1622, and Frankenthal was finally transferred two years later, thus leaving the Palatinate in the hands of the Spaniards.

Question: Which two armies lay siege during the war?
Imperial and the Spanish armies.