Edgie
You can build a scaffold tower from conventional tube and fitting steel components, from a proprietary steel system such as Cuplok or Kwikstage, or from lighweight aluminium.
If it's a lightweight aluminium system then usually access would be up the inside, with an opening with or without hatch at each platform level.
If I understand your posting correctly then the ladder has been put up the outside and is resting on a horizontal rail (ledger or transom) that probably doubles up as a guard-rail, meaning that someone would have to climb over that rail to get onto the platform - not particularly easy getting on to the platform and more difficult to do the reverse.
If so, then the ladder (+ person on it, and anything they are carrying) is applying a lateral force on the tower which it is unlikely to have been designed and erected for [as it would be much easier to provide internal access than to put in place all the structural measures needed to appropriately protect against such lateral forces].
As regards the base, well a tower is usually either mobile and supported by its wheels/castors OR not fitted with wheels/castors and supported by metal base plates that are typically 150mm x 150mm square. What you describe appears to be neither one nor the other.
Beneath that you have referred to decking. May be OK, may be not - depends on the loading bearing down on the decking and what lies beneath that, and, perhaps more crucially, how the load from the scaffold is spread onto the decking and whatever is beneath. Usually, this would be done by putting in spreader boards between base plates and the ground below. So you could e.g. put in a scaffold board each pair of base plates or a board under each base plate - the latter particularly if you need to spread the load further in each horizontal plane.
The spreader boards can also help bridge any weaknesses in the supporting material - you might have a tower built on a street pavement with weak glazing over a basement below, so that the spreader can bridge over the glazing, or in your case, it could spread over weaknesses in the decking, such as the cracks between adjacent parts of the decking.
Lots of guidance on this on the HSE website. www.hse.gov.uk - this will illustrate how you can overcome issues such as lateral loading with e.g. additional bracing and/or outriggers or ties to the adjacent structure.