Possible Clues:

• They enter by force to conquer
• They occupy those who see The Border as something that can be overcome
• Ones storming the castle, say
• Sci-fi villains, often
• Attackers entering another country
• Intruders, conquerers
• Poet of cold, Irish lake
• Gate-crashers' red van is overturned
• Those who occupy dives ran to get drunk
• Mongols, often