Teleportation spells that function for creatures such as Dimension Door, Misty Step, etc. also transport objects that the creatures are wearing or carrying.

As to the source the spell specifically states creature.

A shimmering portal opens within the circle you drew and remains open until the end of your next turn. Any creature that enters the portal instantly appears within 5 feet of the destination circle or in the nearest unoccupied space if that space is occupied.

This is a fairly important distinction as there are other higher lvl spells, specifically Gate, that allow for the transport of anything.

Travel through the portal is possible only by moving through its front. Anything that does so is instantly transported to the other plane, appearing in the unoccupied space nearest to the portal.

That said your strategy might allow for the use of heavily armored kamakazi Kobolds carrying dynamite.

Doesn’t the spell only work on Creatures?

You could table rule this out, but given that your Empire is effectively a nexus city state it could be wiped out fairly quick if the party managed to gain control of said tower or built one of their own. Heck an Illusionist with Mislead and a little luck could probably wipe out the city by impersonating the Emperor.

Hal changes a light bulb.