The Tower is a birthsign that grants a person with the Tower Key power, which allows him or her to open a door or container of "Average" lock level or less once a day. The Tower is one of The Thief's charges and its season is Frostfall. Those born under the sign of the Tower have a knack for finding gold and can open locks of all kinds. The person is also granted The Tower Warden power, which reflects five points of damage for 120 seconds once a day.
At very high levels almost no locks will be average. The once a day power is extremely weak. However, at low levels, it can be very useful when low on lockpicks or doing quests such as Unfriendly Competition which require to open two average locks.
There is also a Tower Stone that grants two powers, the Hand of the Master (+20 armorer for 120 seconds), and the Warden Key power, which can open every lock of "Hard" lock level within 15 feet of the target, once per day. The stone is located just Southwest of the Imperial City Waterfront, on the tip of the small island across from the Waterfront docks.
In the end, this is useful for any character, but only at low level and only situationally. You can get a better birthsign. And you probably should.