The group debated for a while, and considered that the basement route was the more 'known' quantity - they knew the enemy probably considered the slimes sufficient defense for the basement, and would likely be very unprepared for an assault from that direction, while they could have defenses on the upper floors that could react to their presence, especially with a powerful wizard on their side. As they hadn't spent much in the way of resources, they were ready to strike whenever - only Devaguea had used anything in the way of spells.