It's a commonplace of wizard lairs from Azurax Silverhawk to Rary the Traitor and so on that there are special areas where summoning etc work. Look at the 5e version of Mordenkainen's Private Sanctum (PHB p.262) - I don't have my 2E / 3E books to hand - and the caster determines the volume. A skilled caster could cast the spell multiple times to create holes for summoning etc.