There's a hint to a secret in the spoiler tag below and a couple of features that aren't really easter eggs or secret, just not well known.

Not-Really-Secret Features:
1. In the concealed missile silo, if you go into the control room there's two smaller buttons for opening and closing the silo and three larger buttons that control "forcers" that push things up out of the silo. If you put something (or a player...) on one of the yellow and black striped pads and press one of those buttons it will push it fast out of the silo like a missile launch. The amount of white in the buttons indicates how hard the forcers push - the most powerful one will push almost anything to the gmod speed limit and most things will nearly hit the top of the skybox.
Most people also seem to miss that the pipe sticking out of the floor next to the pool has a wheel on top that controls the water level in the pool.

2. In the shooting range there's number pads on the walls of each of the shooting bays to control the enemy spawns. Pressing one starts a 2 minute timer, spawning various enemies that will fight players and each other, including Combine soldiers, Manhacks, Antlions and a Strider. There's also a bunker with ammo, health/suit chargers and mounted guns in the middle of the shooting range that also has a numpad to start the enemy spawns in case you prefer a more up-close fight.
Once the timer expires all the enemies despawn to keep lag to a minimum when running on a server.

Secret Feature
A hint on getting into the tower admin room:
The tower admin room was designed so that there's no direct way in without going via the admin lobby so start by looking for that (look near the room with the anti-gravity chamber). Entry to the room is protected but there is one way in, if you can find it.
From the admin lobby there's a teleporter that takes you to the tower admin room (and a few that...don't).

Hope those give you some fun :P

how go to the tower top?

Originally posted by Я мыл попу:

how go to the tower top?

edited the first post with a hint