I've tried your script:
local blockable_field_items = {1497, 1499, 1496, 1495, 1492, 1493, 1494}
function onUse(cid, item, fromPosition, itemEx, toPosition)
local ladder_destination = {x = toPosition.x, y = toPosition.y + 1, z = toPosition.z - 1}
-- check for players
if...