We recently saw TheViper trying out this fun sneaky strategy in Arena, a map where you’re supposed to feel very safe in the early game. The idea is to maximize the usage of the Slavs' free Supplies (cheaper Militia-line) to amass a large enough army in early Castle Age and deliver it to your opponent’s base via Siege Tower.
The build order is a bit tight as it involves abusing the market to get all your buildings and upgrades as early as possible, while your opponent’s defenses are down. That being said, if you’re playing against a mid-ELO player you can always take it easy and delay your attack by a few minutes. The result should still be very fun and quite powerful.