People always build towers and research 'Sentries' in the hope of having a strong defense system after research completion.
But, a tower without garrisoned soldiers is a present for your enemy. Better destroy your own tower early before letting it fall in the handy of your opponents, if you're not capable of garrisoning your best swords men there.
It's not a bug, it's not unintended, it's part of the players own strategy.