Introducing..... THE COLOSSEUM
The Colosseum is an expensive unit production structure capable of producing two very powerful units.
The Gladiator and the The Gladiator Siege Tower. Expensive, but powerful.
The Gladiator itself has 600 health and is one of the rare units that costs the stone resource.
The Gladiator Siege Tower is the most powerful and expensive Siege Unit in the game.
As you can see, there is plenty of space in the production queue to add new units.