Empires have dominated history and studying them can reveal interesting insights into human nature and the way societies operate. A stimulating way to give pupils an overview is to compare them to one another.
Before embarking on this, use a timeline to plot some of the key empires from history. Then get pupils to compare the periods and make basic assumptions and conclusions - for example, "The Ottoman Empire was the longest serving so probably the most successful."
From this, pupils can brainstorm what makes an empire significant. One aspect you could compare is the effectiveness of armies. Pupils could study the methods of warfare of the Greeks and Romans and comment on the reasons why they were so successful. Research the tactics used by these empires and then take the students into the playground and have a mock battle using what they have researched.
Another aspect is the total landmass the empires covered. By comparing the Roman and British empires you could find out reasons why they were able to conquer and control so many parts of the world. Are there any similarities - for example, use of new naval technology or effective diplomacy?
A fun and engaging summative exercise could be getting pupils to create Top Trump cards about a selection of historical empires. Let them decide the categories to compare - for example, area of land, time in power and people conquered (population).